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/browser_web.dart

@@ -1,3 +1,6 @@
+import 'dart:async';
+import 'dart:js_interop';
+
 import 'package:web/web.dart' as web;
 
 /// Per-tab storage for the PKCE verifier + CSRF state (cleared when the tab
@@ -20,3 +23,41 @@ Map<String, String> currentQueryParameters() =>
 void clearQuery() {
   web.window.history.replaceState(null, '', '/');
 }
+
+/// Opens the OS file picker and reads the chosen file's bytes.
+Future<({String name, List<int> bytes})?> pickFile() {
+  final completer = Completer<({String name, List<int> bytes})?>();
+  final input = web.HTMLInputElement()..type = 'file';
+  input.onchange = ((web.Event _) {
+    final files = input.files;
+    if (files == null || files.length == 0) {
+      completer.complete(null);
+      return;
+    }
+    final file = files.item(0)!;
+    final reader = web.FileReader();
+    reader.onload = ((web.Event _) {
+      final buffer = (reader.result as JSArrayBuffer).toDart;
+      completer.complete((name: file.name, bytes: buffer.asUint8List()));
+    }).toJS;
+    reader.onerror = ((web.Event _) => completer.complete(null)).toJS;
+    reader.readAsArrayBuffer(file);
+  }).toJS;
+  input.click();
+  return completer.future;
+}
+
+/// Triggers a browser download of an in-memory string (e.g. the CSV export).
+void downloadText(
+  String filename,
+  String content, {
+  String mime = 'text/csv;charset=utf-8',
+}) {
+  final blob = web.Blob([content.toJS].toJS, web.BlobPropertyBag(type: mime));
+  final url = web.URL.createObjectURL(blob);
+  web.HTMLAnchorElement()
+    ..href = url
+    ..download = filename
+    ..click();
+  web.URL.revokeObjectURL(url);
+}