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>

backend/src/wahl/dto/update-wahl.dto.ts

@@ -0,0 +1,7 @@
+import { IsBoolean, IsOptional } from 'class-validator';
+
+export class UpdateWahlDto {
+  @IsOptional()
+  @IsBoolean()
+  isOpen?: boolean;
+}