feat(frontend): settings window, i18n, admin UI, books, tasks, calendar, timer

- Settings window with tabs: profile (name, email, avatar, password),
  appearance (light/dark/system), language (de/en/zh), background
  (gradient presets, custom URL, blur toggle); persisted server-side
- i18n dictionaries (de/en/zh) applied across nav, sidebar, tasks,
  calendar, time tracking, admin, invites and comments
- Greeting header with time-of-day message, live clock and date; cooler
  theme, cards, kanban, calendar grid and data-table styling
- Shared sidebar: nav, collapsible books with per-book pages, top-level
  pages, search; user chip opens settings
- Tasks kanban with priorities, overdue flags and book/page links;
  calendar month view fed by task deadlines; time view with entries,
  durations and totals; top-right live timer with optional book link
- User discovery autocomplete in the invite card; editor gains tables
  and checklists; standalone theme buttons removed

frontend/package.json

@@ -10,6 +10,12 @@
   },
   "dependencies": {
     "@tiptap/core": "^2.4.0",
+    "@tiptap/extension-table": "^2.4.0",
+    "@tiptap/extension-table-cell": "^2.4.0",
+    "@tiptap/extension-table-header": "^2.4.0",
+    "@tiptap/extension-table-row": "^2.4.0",
+    "@tiptap/extension-task-item": "^2.4.0",
+    "@tiptap/extension-task-list": "^2.4.0",
     "@tiptap/pm": "^2.4.0",
     "@tiptap/starter-kit": "^2.4.0",
     "@tiptap/vue-3": "^2.4.0",