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/stores/notifications.ts

@@ -0,0 +1,45 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+import {
+  getNotificationCount,
+  getNotifications,
+  markAllNotificationsRead,
+  markNotificationRead,
+  type NotificationEntry,
+} from '@/api/client'
+
+export const useNotificationsStore = defineStore('notifications', () => {
+  const count = ref(0)
+  const items = ref<NotificationEntry[]>([])
+
+  async function fetchCount() {
+    try {
+      count.value = await getNotificationCount()
+    } catch {
+      // silently ignore — polling failure shouldn't surface to user
+    }
+  }
+
+  async function fetchAll() {
+    try {
+      items.value = await getNotifications(false)
+      count.value = items.value.length
+    } catch {
+      //
+    }
+  }
+
+  async function markRead(id: number) {
+    await markNotificationRead(id)
+    items.value = items.value.filter(n => n.id !== id)
+    count.value = Math.max(0, count.value - 1)
+  }
+
+  async function markAll() {
+    await markAllNotificationsRead()
+    items.value = []
+    count.value = 0
+  }
+
+  return { count, items, fetchCount, fetchAll, markRead, markAll }
+})