feat: LT Wahl controls (open/close, Force-Zuteilung, CSV) + file upload

Backend:
- PATCH /api/wahl/:wahlId (isOpen) to open/close a Wahl.
- GET /api/wahl/:wahlId/teilnehmer (LT): participants with their priorities
  and any existing Force-Zuteilung.
- Verified against local Postgres: PATCH toggles isOpen, teilnehmer list
  returns, CSV export works. (A stale dev server on :3000 masked this at
  first — real routes are fine.)

Client (client/app/):
- Wahl detail: open/close switch, participant list with a "Zuteilen"
  (Force-Zuteilung) action, CSV export via a browser download
  (browser.downloadText).
- files_admin_screen.dart: LT file upload — browser.pickFile() +
  visibility picker -> multipart POST /api/files/:kcId; list existing
  files. Reachable from KcDetailScreen.
- browser_web.dart gains pickFile()/downloadText() (native <input file> +
  Blob), with throwing stubs for the VM.
- Dropped the file_picker package again (heavy transitive deps, and the
  native web input is enough); disk on this box is nearly full.

flutter analyze/test/build web green; backend npm test 56.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

client/app/lib/screens/admin_screen.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 
 import '../api.dart';
 import '../main.dart';
+import 'files_admin_screen.dart';
 import 'teamer_admin_screen.dart';
 import 'ui.dart';
 import 'wahl_admin_screen.dart';
@@ -146,6 +147,16 @@ class _KcDetailScreenState extends State<KcDetailScreen> {
               ),
             ),
           ),
+          Card(
+            child: ListTile(
+              leading: const Icon(Icons.folder_shared),
+              title: const Text('Dateien'),
+              trailing: const Icon(Icons.chevron_right),
+              onTap: () => Navigator.of(context).push(
+                MaterialPageRoute(builder: (_) => FilesAdminScreen(kcId: widget.kc.id)),
+              ),
+            ),
+          ),
           const SizedBox(height: 16),
           SectionHeader('Gemeinden', action: TextButton.icon(
             onPressed: _addGemeinde,