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/components/InviteModal.vue

@@ -1,21 +1,33 @@
 <script setup>
 import { ref } from 'vue';
 import api from '../api';
+import { useLocaleStore } from '../stores/locale';
+import UserSearch from './UserSearch.vue';
 
 const props = defineProps({ workspaceId: String });
 const emit = defineEmits(['invited']);
+const L = useLocaleStore();
 const email = ref('');
+const picked = ref(null);
 const role = ref('member');
 const msg = ref('');
 const err = ref('');
 
+function onPick(u) {
+  picked.value = u;
+  email.value = u.email;
+}
+
 async function invite() {
   msg.value = '';
   err.value = '';
+  const target = email.value.trim();
+  if (!target) return;
   try {
-    await api.post(`/workspaces/${props.workspaceId}/members`, { email: email.value, role: role.value });
-    msg.value = `Invited ${email.value} as ${role.value}`;
+    await api.post(`/workspaces/${props.workspaceId}/members`, { email: target, role: role.value });
+    msg.value = `🎉 ${target} → ${role.value}`;
     email.value = '';
+    picked.value = null;
     emit('invited');
   } catch (e) {
     err.value = e.response?.data?.error || 'Invite failed';
@@ -25,17 +37,19 @@ async function invite() {
 
 <template>
   <div class="card">
-    <h4>Invite collaborator</h4>
-    <p class="muted">They must register first with the same email.</p>
-    <div class="row">
+    <h4>{{ L.t('inv.title') }}</h4>
+    <p class="muted">{{ L.t('inv.hint') }}</p>
+    <UserSearch @pick="onPick" />
+    <div class="row" style="margin-top: 8px">
       <input v-model="email" placeholder="teammate@example.com" />
       <select v-model="role" style="max-width: 130px">
         <option value="viewer">viewer</option>
         <option value="member">member</option>
         <option value="admin">admin</option>
       </select>
-      <button @click="invite">Invite</button>
+      <button @click="invite">✉️ {{ L.t('inv.btn') }}</button>
     </div>
+    <p v-if="picked" class="muted">🔎 {{ picked.name }} &lt;{{ picked.email }}&gt;</p>
     <p v-if="msg" style="color: green">{{ msg }}</p>
     <p v-if="err" style="color: red">{{ err }}</p>
   </div>