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/wahl_admin_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 
 import '../api.dart';
+import '../browser.dart' as browser;
 import '../main.dart';
 import 'ui.dart';
 
@@ -92,20 +93,80 @@ class WahlDetailScreen extends StatefulWidget {
 class _WahlDetailScreenState extends State<WahlDetailScreen> {
   Future<List<WorkshopAdmin>>? _workshops;
   Future<List<ZuteilungRow>>? _results;
+  Future<List<TeilnehmerRow>>? _teilnehmer;
+  late bool _isOpen = widget.wahl.isOpen;
+  List<WorkshopAdmin> _workshopCache = const [];
   bool _running = false;
   Api get _api => AppScope.of(context).api;
 
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
-    _workshops ??= _api.workshopsForWahl(widget.wahl.id);
+    _workshops ??= _api.workshopsForWahl(widget.wahl.id).then((w) {
+      _workshopCache = w;
+      return w;
+    });
     _results ??= _api.zuteilungResults(widget.wahl.id);
+    _teilnehmer ??= _api.wahlTeilnehmer(widget.wahl.id);
   }
 
-  void _reloadWorkshops() =>
-      setState(() => _workshops = _api.workshopsForWahl(widget.wahl.id));
+  void _reloadWorkshops() => setState(() {
+        _workshops = _api.workshopsForWahl(widget.wahl.id).then((w) {
+          _workshopCache = w;
+          return w;
+        });
+      });
   void _reloadResults() =>
       setState(() => _results = _api.zuteilungResults(widget.wahl.id));
+  void _reloadTeilnehmer() =>
+      setState(() => _teilnehmer = _api.wahlTeilnehmer(widget.wahl.id));
+
+  Future<void> _toggleOpen(bool value) async {
+    final api = _api;
+    setState(() => _isOpen = value);
+    try {
+      await api.setWahlOpen(widget.wahl.id, value);
+    } catch (e) {
+      if (mounted) {
+        setState(() => _isOpen = !value);
+        toast(context, '$e');
+      }
+    }
+  }
+
+  Future<void> _exportCsv() async {
+    final api = _api;
+    try {
+      final csv = await api.zuteilungCsv(widget.wahl.id);
+      browser.downloadText('zuteilung-${widget.wahl.name}.csv', csv);
+    } catch (e) {
+      if (mounted) toast(context, '$e');
+    }
+  }
+
+  Future<void> _forceFor(TeilnehmerRow t) async {
+    final api = _api;
+    final workshopId = await showDialog<String>(
+      context: context,
+      builder: (_) => SimpleDialog(
+        title: Text('Zuteilung für ${t.name}'),
+        children: [
+          for (final w in _workshopCache)
+            SimpleDialogOption(
+              onPressed: () => Navigator.of(context).pop(w.id),
+              child: Text(w.name),
+            ),
+        ],
+      ),
+    );
+    if (workshopId == null || !mounted) return;
+    try {
+      await api.forceZuteilung(widget.wahl.id, t.id, workshopId);
+      if (mounted) _reloadTeilnehmer();
+    } catch (e) {
+      if (mounted) toast(context, '$e');
+    }
+  }
 
   Future<void> _addWorkshop() async {
     final api = _api;
@@ -142,6 +203,17 @@ class _WahlDetailScreenState extends State<WahlDetailScreen> {
       body: ListView(
         padding: const EdgeInsets.all(16),
         children: [
+          Card(
+            child: SwitchListTile(
+              title: const Text('Wahl geöffnet'),
+              subtitle: Text(_isOpen
+                  ? 'Konfis können Wünsche abgeben'
+                  : 'Geschlossen — keine neuen Einreichungen'),
+              value: _isOpen,
+              onChanged: _toggleOpen,
+            ),
+          ),
+          const SizedBox(height: 8),
           SectionHeader('Workshops', action: TextButton.icon(
             onPressed: _addWorkshop,
             icon: const Icon(Icons.add),
@@ -170,12 +242,47 @@ class _WahlDetailScreenState extends State<WahlDetailScreen> {
             },
           ),
           const Divider(height: 40),
+          SectionHeader('Teilnehmer:innen'),
+          const SizedBox(height: 4),
+          FutureBuilder<List<TeilnehmerRow>>(
+            future: _teilnehmer,
+            builder: (context, snap) {
+              if (snap.connectionState != ConnectionState.done) {
+                return const LinearProgressIndicator();
+              }
+              if (snap.hasError) return Text('Fehler: ${snap.error}');
+              final rows = snap.data!;
+              if (rows.isEmpty) return const Text('Noch keine Einreichungen.');
+              return Column(
+                children: [
+                  for (final t in rows)
+                    ListTile(
+                      dense: true,
+                      leading: const Icon(Icons.person),
+                      title: Text(t.name),
+                      subtitle: Text('Wünsche: ${t.prioritaeten.length}'
+                          '${t.forcedWorkshopId != null ? ' · fest zugeteilt' : ''}'),
+                      trailing: TextButton(
+                        onPressed: () => _forceFor(t),
+                        child: const Text('Zuteilen'),
+                      ),
+                    ),
+                ],
+              );
+            },
+          ),
+          const Divider(height: 40),
           Row(
             children: [
               Expanded(
                 child: Text('Zuteilung',
                     style: Theme.of(context).textTheme.titleMedium),
               ),
+              IconButton(
+                tooltip: 'CSV exportieren',
+                onPressed: _exportCsv,
+                icon: const Icon(Icons.download),
+              ),
               FilledButton.icon(
                 onPressed: _running ? null : _run,
                 icon: _running