Add strikethrough shortcut, fenced code toggle, and CodeMirror-aware undo/redo

- Bind Ctrl+Shift+S to strikethrough
- Smart code toggle: fenced block for multi-line selections, inline otherwise
- Link editing: selecting an existing link now selects its URL for editing
- Route undo/redo shortcuts through IPC so CodeMirror history is used when
  the editor has focus instead of Electron's native menu-role undo/redo
- Add Indent/Outdent toolbar buttons

tests/run-format-test.mjs

@@ -28,6 +28,7 @@ const {
   toggleItalic,
   toggleStrikethrough,
   toggleInlineCode,
+  toggleCode,
   toggleLink,
   setHeading,
   toggleBulletList,
@@ -55,7 +56,12 @@ function target(doc, anchor, head = anchor) {
       return true;
     },
   };
-  return { t, text: () => state.doc.toString(), sel: () => state.selection.main };
+  return {
+    t,
+    text: () => state.doc.toString(),
+    sel: () => state.selection.main,
+    select: (a, h = a) => (state = state.update({ selection: { anchor: a, head: h } }).state),
+  };
 }
 
 let failures = 0;
@@ -147,6 +153,38 @@ function eq(actual, expected, msg) {
   eq(text(), "a[](url)b", "link on empty cursor inserts empty label");
   eq(sel().from, 2, "link cursor inside label brackets");
 }
+{
+  const { t, text, sel } = target("see [docs](old-url) here", 4, 19);
+  toggleLink(t);
+  eq(text(), "see [docs](old-url) here", "link edit does not change text");
+  eq(sel().from, 11, "link edit selects url (from)");
+  eq(sel().to, 18, "link edit selects url (to)");
+}
+
+// ---- code: inline vs fenced ----
+{
+  const { t, text } = target("var x = 1;", 0, 9);
+  toggleCode(t);
+  eq(text(), "`var x = 1`;", "code on single line uses inline markers");
+}
+{
+  const { t, text, select } = target("line one\nline two", 0, 16);
+  toggleCode(t);
+  eq(text(), "```\nline one\nline two\n```", "code on multiple lines uses a fenced block");
+  select(0, 25);
+  toggleCode(t);
+  eq(text(), "line one\nline two", "fenced code toggles back off");
+}
+{
+  const { t, text } = target("before\n```\ncode a\ncode b\n```\nafter", 7, 24);
+  toggleCode(t);
+  eq(text(), "before\ncode a\ncode b\nafter", "fenced code unwraps when selection starts at the fence");
+}
+{
+  const { t, text } = target("abc", 1);
+  toggleCode(t);
+  eq(text(), "a``bc", "code on empty cursor inserts inline markers");
+}
 
 // ---- headings ----
 {

tests/selftest-scenario.js

@@ -141,12 +141,50 @@ const BOOK_DIR = __BOOKDIR_VALUE__;
       await new Promise((r) => setTimeout(r, 50));
       out.redoOk = cm.state.doc.toString().startsWith("## ");
 
+      cm.dispatch({ selection: { anchor: 15, head: 20 } }); // "World"
+      cm.contentDOM.dispatchEvent(
+        new KeyboardEvent("keydown", {
+          key: "s",
+          ctrlKey: true,
+          shiftKey: true,
+          bubbles: true,
+          cancelable: true,
+        })
+      );
+      await new Promise((r) => setTimeout(r, 50));
+      out.strikeShortcutOk = cm.state.doc.toString().includes("~~World~~");
+
+      cm.dispatch({
+        changes: { from: 0, to: 0, insert: "line one\nline two\n" },
+        selection: { anchor: 0, head: 16 },
+      });
+      document.querySelector('[data-cmd="code"]').click();
+      await new Promise((r) => setTimeout(r, 50));
+      out.fencedCodeOk = cm.state.doc.toString().startsWith("```\nline one\nline two\n```");
+
       await new Promise((r) => setTimeout(r, 1200)); // wait for autosave debounce
       out.formattingAutosaved =
         (await window.folio.getChapterContent(typedId)).includes("**Hello**");
     }
 
-    out.all = true;
+    const okFlags = [
+      "dndMoved",
+      "kbMoved",
+      "modalShown",
+      "uiCreated",
+      "hasEditor",
+      "editorAutosaved",
+      "toolbarBoldOk",
+      "focusAfterBold",
+      "shortcutBoldOk",
+      "headingOk",
+      "undoOk",
+      "redoOk",
+      "strikeShortcutOk",
+      "fencedCodeOk",
+      "formattingAutosaved",
+    ];
+    out.all = okFlags.every((f) => out[f] === true);
   } catch (e) {
     out.error = String((e && e.message) || e);
   }