feat: add notifications and project shares features

- Backend: notifications + shares routes, DB tables, SqliteAdapter methods
- Frontend: NotificationPanel (bell icon + dropdown), ShareDialog, Dashboard integration
- Styles: notification + share dialog CSS

backend/src/database/SqliteAdapter.ts

@@ -9,6 +9,7 @@ import { randomUUID } from 'crypto';
 import type {
   DatabaseInterface, DBProject, DBDrawing, DBLayer,
   DBElement, DBBlock, DBSetting, DBUser, DBSession,
+  DBNotification, DBProjectShare,
 } from './DatabaseInterface.js';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
@@ -265,4 +266,43 @@ export class SqliteAdapter implements DatabaseInterface {
   deleteExpiredSessions(): void {
     this.db.prepare('DELETE FROM sessions WHERE expires_at < ?').run(Date.now());
   }
+
+  // ─── Notifications ──────────────────────────────────
+  listNotifications(userId: string): DBNotification[] {
+    return this.db.prepare('SELECT * FROM notifications WHERE user_id = ? ORDER BY created_at DESC').all(userId) as DBNotification[];
+  }
+
+  createNotification(data: Partial<DBNotification>): DBNotification {
+    const id = data.id ?? `notif-${Date.now()}`;
+    this.db.prepare(
+      'INSERT INTO notifications (id, user_id, type, title, message, read) VALUES (?, ?, ?, ?, ?, ?)',
+    ).run(id, data.user_id!, data.type ?? 'info', data.title!, data.message!, data.read ?? 0);
+    return this.db.prepare('SELECT * FROM notifications WHERE id = ?').get(id) as DBNotification;
+  }
+
+  markNotificationRead(id: string): boolean {
+    return this.db.prepare('UPDATE notifications SET read = 1 WHERE id = ?').run(id).changes > 0;
+  }
+
+  deleteNotification(id: string): boolean {
+    return this.db.prepare('DELETE FROM notifications WHERE id = ?').run(id).changes > 0;
+  }
+
+  // ─── Project Shares ─────────────────────────────────
+  listProjectShares(projectId: string): DBProjectShare[] {
+    return this.db.prepare('SELECT * FROM project_shares WHERE project_id = ? ORDER BY created_at DESC').all(projectId) as DBProjectShare[];
+  }
+
+  createProjectShare(data: Partial<DBProjectShare>): DBProjectShare {
+    const id = data.id ?? `share-${Date.now()}`;
+    const shareToken = data.share_token ?? randomUUID();
+    this.db.prepare(
+      'INSERT INTO project_shares (id, project_id, shared_with_email, shared_by, permission, share_token) VALUES (?, ?, ?, ?, ?, ?)',
+    ).run(id, data.project_id!, data.shared_with_email!, data.shared_by!, data.permission ?? 'view', shareToken);
+    return this.db.prepare('SELECT * FROM project_shares WHERE id = ?').get(id) as DBProjectShare;
+  }
+
+  deleteProjectShare(id: string): boolean {
+    return this.db.prepare('DELETE FROM project_shares WHERE id = ?').run(id).changes > 0;
+  }
 }