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

client/src/api/client.ts

@@ -184,6 +184,8 @@ export const adminApi = {
     apiClient.get('/admin/audit-log', { params }).then(r => r.data),
   mcpTokens: () => apiClient.get('/admin/mcp-tokens').then(r => r.data),
   deleteMcpToken: (id: number) => apiClient.delete(`/admin/mcp-tokens/${id}`).then(r => r.data),
+  getPermissions: () => apiClient.get('/admin/permissions').then(r => r.data),
+  updatePermissions: (permissions: Record<string, string>) => apiClient.put('/admin/permissions', { permissions }).then(r => r.data),
 }
 
 export const addonsApi = {