Add chapter color customization

- Store an optional accent color on each ChapterEntry so it persists with the
  chapter across switching, reordering, close/reopen, and restart
- Add a per-row color button that opens a palette popover (assign / change /
  remove) and shows a small color dot in the sidebar, readable in Light/Dark
- Expose setChapterColor through the main/preload IPC bridge

src/main/chapters.ts

@@ -125,6 +125,22 @@ export function renameChapter(bookPath: string, id: string, newTitle: string): v
   saveMeta(bookPath, meta);
 }
 
+// Assign, change, or remove (null) a chapter's accent color. The color lives on
+// the ChapterEntry, so it moves with the chapter on reorder and persists in the
+// book metadata. Unknown ids throw so the caller can surface a clear error.
+export function setChapterColor(
+  bookPath: string,
+  id: string,
+  color: string | null
+): void {
+  const meta = loadBook(bookPath);
+  const entry = meta.chapters[id];
+  if (!entry) throw new Error("Chapter not found");
+  if (color === null) delete entry.color;
+  else entry.color = color;
+  saveMeta(bookPath, meta);
+}
+
 export function deleteChapter(bookPath: string, id: string): void {
   const meta = loadBook(bookPath);
   const entry = meta.chapters[id];

src/main/index.ts

@@ -18,6 +18,7 @@ import {
   duplicateChapter,
   getChapterContent,
   renameChapter,
+  setChapterColor,
   reorderChapters,
   listChaptersInOrder,
   setChapterContent,
@@ -273,6 +274,16 @@ ipcMain.handle("folio:duplicateChapter", (_e, id: string, title?: string) => {
   }
 });
 
+ipcMain.handle("folio:setChapterColor", (_e, id: string, color: string | null) => {
+  try {
+    setChapterColor(requireBook(), id, color);
+    notifyChange();
+    return { ok: true };
+  } catch (e) {
+    return { error: (e as Error).message };
+  }
+});
+
 ipcMain.handle("folio:reorderChapters", (_e, newOrder: string[]) => {
   try {
     reorderChapters(requireBook(), newOrder);

src/main/preload.ts

@@ -19,6 +19,8 @@ contextBridge.exposeInMainWorld("folio", {
     ipcRenderer.invoke("folio:createChapter", title),
   renameChapter: (id: string, title: string) =>
     ipcRenderer.invoke("folio:renameChapter", id, title),
+  setChapterColor: (id: string, color: string | null) =>
+    ipcRenderer.invoke("folio:setChapterColor", id, color),
   deleteChapter: (id: string) =>
     ipcRenderer.invoke("folio:deleteChapter", id),
   duplicateChapter: (id: string, title?: string) =>

src/main/project.ts

@@ -5,6 +5,9 @@ export interface ChapterEntry {
   id: string;
   title: string;
   file: string;
+  // Optional accent color (hex string) shown as a sidebar indicator. Stored on
+  // the chapter itself so it travels with reordering and persists in folio.json.
+  color?: string;
 }
 
 export interface BookMeta {

src/renderer/book.ts

@@ -30,6 +30,7 @@ export type FolioAPI = {
   getChapterContent: (id: string) => Promise<string | { error: string }>;
   createChapter: (title: string) => Promise<any>;
   renameChapter: (id: string, title: string) => Promise<any>;
+  setChapterColor: (id: string, color: string | null) => Promise<any>;
   deleteChapter: (id: string) => Promise<any>;
   duplicateChapter: (id: string, title?: string) => Promise<any>;
   reorderChapters: (newOrder: string[]) => Promise<any>;
@@ -74,6 +75,7 @@ interface ChapterEntry {
   id: string;
   title: string;
   file: string;
+  color?: string;
 }
 
 interface Recent {
@@ -578,6 +580,105 @@ export function initBook(api: FolioAPI) {
     }
   }
 
+  // Predefined palette used for the chapter-color picker (kept in sync with the
+  // text/highlight palettes so the whole app feels consistent).
+  const CHAPTER_COLORS = [
+    "#e11d48", "#f97316", "#eab308", "#22c55e",
+    "#3b82f6", "#8b5cf6", "#78716c", "#18181b",
+  ];
+  const HEX_RE = /^#[0-9a-fA-F]{3,8}$/;
+
+  let colorPopover: HTMLDivElement | null = null;
+  function closeColorPopover() {
+    if (colorPopover) colorPopover.classList.add("hidden");
+  }
+
+  function openColorPopover(
+    anchor: HTMLElement,
+    current: string | null,
+    onPick: (c: string | null) => void
+  ) {
+    if (!colorPopover) {
+      colorPopover = document.createElement("div");
+      colorPopover.className = "color-popover hidden";
+      document.body.appendChild(colorPopover);
+    }
+    const pop = colorPopover;
+    pop.innerHTML = "";
+    const grid = document.createElement("div");
+    grid.className = "cp-grid";
+    for (const c of CHAPTER_COLORS) {
+      const sw = document.createElement("button");
+      sw.type = "button";
+      sw.className = "cp-swatch" + (current === c ? " active" : "");
+      sw.style.background = c;
+      sw.title = c;
+      sw.setAttribute("aria-label", c);
+      sw.addEventListener("click", (e) => {
+        e.stopPropagation();
+        closeColorPopover();
+        onPick(c);
+      });
+      grid.appendChild(sw);
+    }
+    pop.appendChild(grid);
+    const remove = document.createElement("button");
+    remove.type = "button";
+    remove.className = "cp-remove";
+    remove.textContent = current ? "Remove color" : "No color set";
+    remove.disabled = !current;
+    remove.addEventListener("click", (e) => {
+      e.stopPropagation();
+      closeColorPopover();
+      onPick(null);
+    });
+    pop.appendChild(remove);
+
+    pop.classList.remove("hidden");
+    const pw = pop.offsetWidth;
+    const ph = pop.offsetHeight;
+    const rect = anchor.getBoundingClientRect();
+    let left = rect.left;
+    let top = rect.bottom + 4;
+    if (left + pw > window.innerWidth - 8) {
+      left = Math.max(8, window.innerWidth - pw - 8);
+    }
+    if (top + ph > window.innerHeight - 8) {
+      top = Math.max(8, rect.top - ph - 4);
+    }
+    pop.style.left = `${left}px`;
+    pop.style.top = `${top}px`;
+  }
+
+  // Click outside the popover closes it (the open button stops propagation so it
+  // doesn't immediately self-close).
+  document.addEventListener("click", (e) => {
+    if (colorPopover && !colorPopover.classList.contains("hidden") &&
+        !colorPopover.contains(e.target as Node)) {
+      closeColorPopover();
+    }
+  });
+  window.addEventListener("scroll", closeColorPopover, true);
+  window.addEventListener("resize", closeColorPopover);
+
+  async function applyChapterColor(id: string, color: string | null) {
+    const res = await api.setChapterColor(id, color);
+    if (res && "error" in res) {
+      await alertMessage("Could not set chapter color", (res as { error: string }).error);
+      return;
+    }
+    const entry = ordered.find((c) => c.id === id);
+    if (entry) {
+      if (color === null) delete entry.color;
+      else entry.color = color;
+    }
+    if (meta && meta.chapters[id]) {
+      if (color === null) delete meta.chapters[id].color;
+      else meta.chapters[id].color = color;
+    }
+    renderChapterList();
+  }
+
   function renderChapterList() {
     chapterList.innerHTML = "";
     for (const ch of ordered) {
@@ -590,10 +691,18 @@ export function initBook(api: FolioAPI) {
         <span class="drag-handle" title="Drag to reorder">⋮</span>
         <span class="row-title" title="${ch.title}">${ch.title}</span>
         <span class="row-actions">
+          <button class="icon-btn" data-act="color" title="Chapter color">●</button>
           <button class="icon-btn" data-act="rename" title="Rename">✎</button>
           <button class="icon-btn" data-act="dup" title="Duplicate">⧉</button>
           <button class="icon-btn" data-act="del" title="Delete">×</button>
         </span>`;
+      if (ch.color && HEX_RE.test(ch.color)) {
+        const dot = document.createElement("span");
+        dot.className = "row-color-dot";
+        dot.style.background = ch.color;
+        dot.title = "Chapter color: " + ch.color;
+        li.insertBefore(dot, li.querySelector(".row-title"));
+      }
       li.addEventListener("click", (e) => {
         if (dragJustHappened) {
           dragJustHappened = false;
@@ -604,6 +713,10 @@ export function initBook(api: FolioAPI) {
         li.focus();
         void selectChapter(ch.id);
       });
+      li.querySelector('[data-act="color"]')?.addEventListener("click", (e) => {
+        e.stopPropagation();
+        openColorPopover(li, ch.color ?? null, (c) => void applyChapterColor(ch.id, c));
+      });
       li.querySelector('[data-act="rename"]')?.addEventListener("click", (e) => {
         e.stopPropagation();
         void rename(ch.id);

src/renderer/index.css

@@ -526,6 +526,67 @@ body.dragging-active * {
   border-color: var(--color-on-accent-border);
 }
 
+/* Small accent dot shown next to a chapter's title when it has a color. */
+.row-color-dot {
+  flex: 0 0 auto;
+  width: 10px;
+  height: 10px;
+  border-radius: 50%;
+  margin-right: 6px;
+  box-shadow: inset 0 0 0 1px rgba(0, 0, 0, 0.25);
+}
+
+/* Chapter color picker popover. */
+.color-popover {
+  position: fixed;
+  z-index: 1000;
+  background: var(--color-panel);
+  color: var(--color-text);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius);
+  padding: 8px;
+  box-shadow: 0 8px 24px var(--color-shadow-strong);
+}
+.color-popover.hidden {
+  display: none;
+}
+.color-popover .cp-grid {
+  display: grid;
+  grid-template-columns: repeat(4, 1fr);
+  gap: 6px;
+  margin-bottom: 6px;
+}
+.color-popover .cp-swatch {
+  width: 22px;
+  height: 22px;
+  border-radius: 50%;
+  border: 1px solid rgba(0, 0, 0, 0.25);
+  cursor: pointer;
+  padding: 0;
+  transition: transform 0.1s ease;
+}
+.color-popover .cp-swatch:hover {
+  transform: scale(1.12);
+}
+.color-popover .cp-swatch.active {
+  outline: 2px solid var(--color-text);
+  outline-offset: 1px;
+}
+.color-popover .cp-remove {
+  width: 100%;
+  border: 1px solid var(--color-border);
+  background: transparent;
+  color: var(--color-text);
+  border-radius: calc(var(--radius) - 4px);
+  padding: 4px 6px;
+  font-size: 0.8rem;
+  cursor: pointer;
+}
+.color-popover .cp-remove:disabled {
+  opacity: 0.5;
+  cursor: default;
+}
+
 /* ---- chapter view ---- */
 .chapter-view {
   flex: 1;

tests/run-chapters-test.mjs

@@ -39,6 +39,7 @@ const {
   duplicateChapter,
   reorderChapters,
   setChapterContent,
+  setChapterColor,
   moveInOrder,
 } = chapters;
 
@@ -109,6 +110,30 @@ threw = false;
 try { reorderChapters(dir, ["bogus", order[0]]); } catch { threw = true; }
 ok(threw, "reorder throws on invalid id");
 
+// ---- chapter color ----
+const col = createChapter(dir, "Colorful");
+setChapterColor(dir, col.id, "#22c55e");
+ok(loadBook(dir).chapters[col.id].color === "#22c55e", "setChapterColor assigns color");
+setChapterColor(dir, col.id, "#3b82f6");
+ok(loadBook(dir).chapters[col.id].color === "#3b82f6", "setChapterColor changes color");
+setChapterColor(dir, col.id, null);
+ok(!("color" in loadBook(dir).chapters[col.id]), "setChapterColor removes color (null)");
+
+// color travels with the chapter on reorder
+setChapterColor(dir, col.id, "#e11d48");
+const before = loadBook(dir).chapterOrder.slice();
+reorderChapters(dir, [before[before.length - 1], ...before.slice(0, -1)]);
+ok(loadBook(dir).chapters[col.id].color === "#e11d48", "color persists after reorder");
+
+// duplicate does not inherit the source color
+const colDup = duplicateChapter(dir, col.id);
+ok(!("color" in loadBook(dir).chapters[colDup.id]), "duplicate starts without color");
+
+// setChapterColor throws on unknown id
+threw = false;
+try { setChapterColor(dir, "nope", "#fff"); } catch { threw = true; }
+ok(threw, "setChapterColor throws on unknown id");
+
 // empty title throws
 threw = false;
 try { createChapter(dir, "   "); } catch { threw = true; }