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

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; }