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/components/Files/FileManager.tsx

@@ -6,6 +6,8 @@ import { useToast } from '../shared/Toast'
 import { useTranslation } from '../../i18n'
 import { filesApi } from '../../api/client'
 import type { Place, Reservation, TripFile, Day, AssignmentsMap } from '../../types'
+import { useCanDo } from '../../store/permissionsStore'
+import { useTripStore } from '../../store/tripStore'
 
 function authUrl(url: string): string {
   const token = localStorage.getItem('auth_token')
@@ -159,6 +161,8 @@ export default function FileManager({ files = [], onUpload, onDelete, onUpdate,
   const [trashFiles, setTrashFiles] = useState<TripFile[]>([])
   const [loadingTrash, setLoadingTrash] = useState(false)
   const toast = useToast()
+  const can = useCanDo()
+  const trip = useTripStore((s) => s.trip)
   const { t, locale } = useTranslation()
 
   const loadTrash = useCallback(async () => {
@@ -710,7 +714,7 @@ export default function FileManager({ files = [], onUpload, onDelete, onUpdate,
       ) : (
         <>
           {/* Upload zone */}
-          <div
+          {can('file_upload', trip) && <div
             {...getRootProps()}
             style={{
               margin: '16px 16px 0', border: '2px dashed', borderRadius: 14, padding: '20px 16px',
@@ -735,7 +739,7 @@ export default function FileManager({ files = [], onUpload, onDelete, onUpdate,
                 </p>
               </>
             )}
-          </div>
+          </div>}
 
           {/* Filter tabs */}
           <div style={{ display: 'flex', gap: 4, padding: '12px 16px 0', flexShrink: 0, flexWrap: 'wrap' }}>