feat: add configurable permissions system with admin panel

Adds a full permissions management feature allowing admins to control
who can perform actions across the app (trip CRUD, files, places,
budget, packing, reservations, collab, members, share links).

- New server/src/services/permissions.ts: 16 configurable actions,
  in-memory cache, checkPermission() helper, backwards-compatible
  defaults matching upstream behaviour
- GET/PUT /admin/permissions endpoints; permissions loaded into
  app-config response so clients have them on startup
- checkPermission() applied to all mutating route handlers across
  10 server route files; getTripOwnerId() helper eliminates repeated
  inline DB queries; trips.ts and files.ts now reuse canAccessTrip()
  result to avoid redundant DB round-trips
- New client/src/store/permissionsStore.ts: Zustand store +
  useCanDo() hook; TripOwnerContext type accepts both Trip and
  DashboardTrip shapes without casting at call sites
- New client/src/components/Admin/PermissionsPanel.tsx: categorised
  UI with per-action dropdowns, customised badge, save/reset
- AdminPage, DashboardPage, FileManager, PlacesSidebar,
  TripMembersModal gated via useCanDo(); no prop drilling
- 46 perm.* translation keys added to all 12 language files

server/src/routes/auth.ts

@@ -13,6 +13,7 @@ import { db } from '../db/database';
 import { authenticate, demoUploadBlock } from '../middleware/auth';
 import { JWT_SECRET } from '../config';
 import { encryptMfaSecret, decryptMfaSecret } from '../services/mfaCrypto';
+import { getAllPermissions } from '../services/permissions';
 import { randomBytes, createHash } from 'crypto';
 import { revokeUserSessions } from '../mcp';
 import { AuthRequest, User } from '../types';
@@ -209,6 +210,7 @@ router.get('/app-config', (_req: Request, res: Response) => {
     timezone: process.env.TZ || Intl.DateTimeFormat().resolvedOptions().timeZone || 'UTC',
     notification_channel: notifChannel,
     trip_reminders_enabled: tripRemindersEnabled,
+    permissions: getAllPermissions(),
   });
 });