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/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);
   }