Add Undo for book and chapter deletions

- Show an Undo toast after deleting a book from Home; Undo restores the
  folder from the system trash (XDG .trashinfo, URL-decoded path match).
- Chapter delete/cut now snapshots the chapter (title, color, content,
  position) and offers Undo to restore it in place.
- Add reusable showUndoToast/hideUndoToast and #undoToast UI + styles;
  expose restoreBook/restoreChapter over the preload bridge.
- Drop the now-redundant 'cannot be undone' confirm on chapter delete and
  the inaccurate 'permanently removes' wording on book delete.

tests/selftest-scenario.js

@@ -170,7 +170,33 @@
         }
         step("exports", ex);
 
-        // New book auto-creates a folder inside Documents; then delete it.
+        // Chapter delete + undo (snapshot restore).
+        try {
+          const before = (await api.getChapterList()).length;
+          const cc = await api.getChapterContent("ch-one");
+          const delc = await api.deleteChapter("ch-one");
+          const afterDel = (await api.getChapterList()).length;
+          const restc = await api.restoreChapter({
+            id: "ch-one",
+            title: "Book_The_Lab",
+            color: null,
+            content: cc,
+            index: 0,
+          });
+          const afterRest = (await api.getChapterList()).length;
+          const cc2 = await api.getChapterContent("ch-one");
+          step("chapterUndo", {
+            before,
+            afterDel,
+            afterRest,
+            restored: cc2 === cc,
+            ok: !!(delc && delc.ok) && !!(restc && restc.ok),
+          });
+        } catch (e) {
+          step("chapterUndoErr", String(e));
+        }
+
+        // New book auto-creates a folder inside Documents; delete then undo.
         const nb = await api.newBookNamed("Folio Selftest Temp");
         step("newBookNamed", {
           ok: !!(nb && nb.path && nb.title),
@@ -180,6 +206,10 @@
         if (nb && nb.path) {
           const del = await api.deleteBook(nb.path);
           step("deleteBook", del && del.ok ? "ok" : del && del.error);
+          if (del && del.ok) {
+            const rest = await api.restoreBook(nb.path);
+            step("restoreBook", rest && rest.ok ? "ok" : rest && rest.error);
+          }
         }
 
         out.ok = true;