Multi-user sharing, groups, and in-app notifications

Backend:
- Alembic migration 0025: groups, group_memberships, project_shares,
  note_shares, notifications tables
- models: Group, GroupMembership, ProjectShare, NoteShare, Notification
- services/access.py: permission resolution (viewer/editor/admin/owner)
- services/groups.py + routes/groups.py: full group CRUD + membership
- services/sharing.py + routes/shares.py: project/note sharing API
- services/notifications.py: in-app notification create/list/mark-read
- routes/in_app_notifications.py: GET/POST notification endpoints
- routes/users.py: user search endpoint
- services/projects.py + services/notes.py: *_for_user variants
- routes updated to use *_for_user on get/list; adds permission field
- app.py: register all new blueprints

Frontend:
- api/client.ts: ShareEntry, GroupEntry, NotificationEntry types + helpers
- stores/notifications.ts: Pinia store with polling
- NotificationBell.vue: bell icon + badge + dropdown toggle + 60s poll
- NotificationsPanel.vue: unread notification list with mark-all-read
- ShareDialog.vue: teleport modal for sharing projects/notes with users/groups
- SharedWithMeView.vue: /shared route listing shared projects and notes
- AppHeader: NotificationBell, Shared nav link
- ProjectView, NoteViewerView, TaskViewerView: Share button + ShareDialog
- SettingsView: Groups admin tab with create/delete groups + member mgmt
- router: /shared route

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/AppHeader.vue

@@ -6,6 +6,7 @@ import { useShortcuts } from "@/composables/useShortcuts";
 import { useAuthStore } from "@/stores/auth";
 import { useChatStore } from "@/stores/chat";
 import AppLogo from "@/components/AppLogo.vue";
+import NotificationBell from "@/components/NotificationBell.vue";
 
 const { theme, toggleTheme } = useTheme();
 const { toggleShortcuts } = useShortcuts();
@@ -76,6 +77,7 @@ router.afterEach(() => {
         <router-link to="/tasks" class="nav-link">Tasks</router-link>
         <router-link to="/chat" :class="['nav-link', { 'router-link-active': isChatActive }]">Chat</router-link>
         <router-link to="/graph" class="nav-link">Graph</router-link>
+        <router-link to="/shared" class="nav-link">Shared</router-link>
       </div>
 
       <!-- Right: status + utilities + gear + user -->
@@ -85,6 +87,8 @@ router.afterEach(() => {
           <span class="status-text">{{ statusShortLabel }}</span>
         </span>
 
+        <NotificationBell />
+
         <button class="btn-icon" @click="toggleShortcuts" title="Keyboard shortcuts (?)">?</button>
 
         <button class="btn-icon" @click="toggleTheme" :title="`Switch to ${theme === 'dark' ? 'light' : 'dark'} mode`">
@@ -121,6 +125,7 @@ router.afterEach(() => {
       <router-link to="/tasks" class="nav-link">Tasks</router-link>
       <router-link to="/chat" :class="['nav-link', { 'router-link-active': isChatActive }]">Chat</router-link>
       <router-link to="/graph" class="nav-link">Graph</router-link>
+      <router-link to="/shared" class="nav-link">Shared</router-link>
       <div class="mobile-divider"></div>
       <router-link to="/settings" class="nav-link">Settings</router-link>
       <div class="mobile-divider"></div>