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

docs/API.md

@@ -59,6 +59,32 @@ Content fields: `content_json` (TipTap JSON), `content_html` (rendered HTML).
 ## Users
 
 - `GET /users/search?email=` — lookup for invites (auth required).
+- `GET /users/discover?q=` — fuzzy discovery (min 2 chars, max 10 hits, no hashes).
+- `PATCH /users/me` — profile/appearance (`name, email, avatar, locale, theme, bg_image, bg_blur`).
+- `PATCH /users/me/password` — `{ currentPassword, newPassword }`.
+
+## Books
+
+- `GET /books/by-workspace/:workspaceId`, `POST /books {workspace_id,title?,icon?}`
+- `PATCH /books/:id`, `DELETE /books/:id` (pages inside cascade).
+
+## Tasks
+
+- `GET /tasks/by-workspace/:workspaceId?status=&from=&to=` (calendar range on `due_at`)
+- `POST /tasks {workspace_id, book_id?, page_id?, title, status?, priority?, due_at?}`
+- `PATCH /tasks/:id`, `DELETE /tasks/:id`
+
+## Time tracking
+
+- `GET /time/by-workspace/:workspaceId` → `{ entries, open }`
+- `POST /time/start {workspace_id, book_id?, page_id?, description?}` (409 if running)
+- `POST /time/stop {workspace_id}` → sets `ended_at` + `duration_seconds`
+- `PATCH /time/:id`, `DELETE /time/:id` (own or admin+)
+
+## Admin (requires `is_admin`)
+
+- `GET /admin/overview` → counts + recent users
+- `GET /admin/users`, `PATCH /admin/users/:id {is_admin}`, `DELETE /admin/users/:id`
 
 ## Health