feat: RBAC, atomic deployments, and fleet-wide backups (Pro) (#181)

* feat: add RBAC viewer accounts, atomic deployments, and fleet-wide backups (Pro)

Introduces three Pro-tier features:

- RBAC: Multi-user system with admin/viewer roles, user management UI,
  automatic migration from single-admin credentials, viewer restrictions
  across the entire UI (read-only editor, hidden action buttons)

- Atomic Deployments: Pre-deploy file backup to .sencho-backup/, automatic
  rollback on health probe failure, manual rollback button, health probes
  added to stack updates, webhook-triggered deploys use atomic rollback

- Fleet-Wide Backups: Point-in-time snapshots of compose files across all
  nodes (local + remote), stored centrally in SQLite, per-stack restore
  with optional redeploy, graceful handling of offline nodes

* fix(settings): use correct ProGate prop name in UsersSection

* fix(settings): remove unused isPro prop from UsersSection

* fix(auth): fetch user info after login and setup so isAdmin is set correctly

backend/src/__tests__/helpers/setupTestDb.ts

@@ -34,6 +34,9 @@ export async function setupTestDb(): Promise<string> {
   db.updateGlobalSetting('auth_password_hash', passwordHash);
   db.updateGlobalSetting('auth_jwt_secret', TEST_JWT_SECRET);
 
+  // Also seed the users table (RBAC login reads from here)
+  db.addUser({ username: TEST_USERNAME, password_hash: passwordHash, role: 'admin' });
+
   return tmpDir;
 }