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/src/stores/time.js

@@ -0,0 +1,52 @@
+import { defineStore } from 'pinia';
+import api from '../api';
+
+export function formatDuration(totalSeconds) {
+  const s = Math.max(0, Math.floor(totalSeconds || 0));
+  const h = String(Math.floor(s / 3600)).padStart(2, '0');
+  const m = String(Math.floor((s % 3600) / 60)).padStart(2, '0');
+  const sec = String(s % 60).padStart(2, '0');
+  return `${h}:${m}:${sec}`;
+}
+
+export const useTimeStore = defineStore('time', {
+  state: () => ({
+    entries: [],
+    open: null, // running timer entry (ended_at == null)
+    workspaceId: null,
+  }),
+  getters: {
+    elapsed: (s) => {
+      if (!s.open) return 0;
+      return (Date.now() - new Date(s.open.started_at).getTime()) / 1000;
+    },
+  },
+  actions: {
+    async fetch(workspaceId) {
+      this.workspaceId = workspaceId;
+      const { data } = await api.get(`/time/by-workspace/${workspaceId}`);
+      this.entries = data.entries;
+      this.open = data.open;
+    },
+    async start(description = '', book_id = null) {
+      const { data } = await api.post('/time/start', {
+        workspace_id: this.workspaceId, description, book_id,
+      });
+      this.open = data.entry;
+      this.entries.unshift(data.entry);
+      return data.entry;
+    },
+    async stop() {
+      const { data } = await api.post('/time/stop', { workspace_id: this.workspaceId });
+      this.open = null;
+      const idx = this.entries.findIndex((e) => e.id === data.entry.id);
+      if (idx >= 0) this.entries[idx] = { ...data.entry, user_name: this.entries[idx].user_name };
+      else this.entries.unshift(data.entry);
+      return data.entry;
+    },
+    async remove(id) {
+      await api.delete(`/time/${id}`);
+      this.entries = this.entries.filter((e) => e.id !== id);
+    },
+  },
+});