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/views/NoteViewerView.vue

@@ -8,12 +8,14 @@ import { apiPost, apiGet } from "@/api/client";
 import type { Note } from "@/types/note";
 import TagPill from "@/components/TagPill.vue";
 import TableOfContents from "@/components/TableOfContents.vue";
+import ShareDialog from "@/components/ShareDialog.vue";
 
 const route = useRoute();
 const router = useRouter();
 const store = useNotesStore();
 const backlinks = ref<{ type: string; id: number; title: string }[]>([]);
 const converting = ref(false);
+const showShare = ref(false);
 
 // Context enrichment
 const projectTitle = ref<string | null>(null);
@@ -169,6 +171,7 @@ async function convertToTask() {
           >
             {{ converting ? "Converting..." : "Convert to Task" }}
           </button>
+          <button class="btn-share" @click="showShare = true">Share</button>
         </div>
 
         <!-- Breadcrumb: parent → project → milestone -->
@@ -248,6 +251,14 @@ async function convertToTask() {
       class="toc-sidebar"
     />
   </div>
+
+  <ShareDialog
+    v-if="showShare && store.currentNote"
+    resource-type="note"
+    :resource-id="store.currentNote.id"
+    :resource-title="store.currentNote.title || '(untitled)'"
+    @close="showShare = false"
+  />
 </template>
 
 <style src="@/assets/viewer-shared.css" />
@@ -334,6 +345,19 @@ async function convertToTask() {
   cursor: default;
 }
 
+.btn-share {
+  padding: 0.3rem 0.75rem;
+  background: var(--color-bg-secondary);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
+  color: var(--color-text-secondary);
+  cursor: pointer;
+  font-size: 0.85rem;
+  font-family: inherit;
+  transition: border-color 0.15s, color 0.15s;
+}
+.btn-share:hover { border-color: var(--color-primary); color: var(--color-primary); }
+
 .note-title {
   font-family: "Fraunces", Georgia, serif;
   font-size: 2rem;