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/App.vue

@@ -1,17 +1,40 @@
 <script setup>
-import { onMounted } from 'vue';
+import { computed, onMounted } from 'vue';
+import { useAuthStore } from './stores/auth';
 import { useThemeStore } from './stores/theme';
-import ThemeToggle from './components/ThemeToggle.vue';
+import { useLocaleStore } from './stores/locale';
+import { useUiStore } from './stores/ui';
+import SettingsWindow from './components/SettingsWindow.vue';
+import TimeTrackerButton from './components/TimeTrackerButton.vue';
 
+const auth = useAuthStore();
 const theme = useThemeStore();
-onMounted(() => theme.apply());
+const locale = useLocaleStore();
+const ui = useUiStore();
+
+const bgStyle = computed(() => {
+  const bg = auth.user?.bgImage || '';
+  if (!bg) return {};
+  const style = { backgroundImage: bg.startsWith('http') ? `url("${bg}")` : bg };
+  if (auth.user?.bgBlur !== false) style.filter = 'blur(14px) brightness(0.85)';
+  return style;
+});
+
+onMounted(() => {
+  locale.set(locale.locale);
+  theme.apply();
+  auth.applyPrefs();
+});
 </script>
 
 <template>
   <div>
-    <div style="position: fixed; top: 12px; right: 12px; z-index: 50">
-      <ThemeToggle />
+    <div v-if="bgStyle.backgroundImage" class="bg-layer" :style="bgStyle"></div>
+    <div v-if="auth.isLoggedIn" class="top-right">
+      <TimeTrackerButton />
+      <button class="ghost icon-btn" @click="ui.openSettings()" title="⚙️">⚙️</button>
     </div>
     <router-view />
+    <SettingsWindow />
   </div>
 </template>