feat(client): LT Wahl admin, Teamer admin, Verantwortlichen self-registration

New screens (client/app/lib/screens/):
- wahl_admin_screen.dart — per KC: list/create Wahlen; per Wahl: add
  workshops, run the assignment (POST /wahl/:id/zuteilung/run), view the
  result table.
- teamer_admin_screen.dart — per Gemeinde: list/create local Teamer
  accounts, create group-link or per-email invites (shows the token).
- verantwortliche_register_screen.dart — enter a KC invite code
  (GET /onboarding/kc/:code), pick a Gemeinde, submit
  (POST /onboarding/verantwortliche); shown on the home screen to a
  logged-in Authentik user who has no membership yet.
- ui.dart — shared toast / ErrorText / SectionHeader / promptText.
KcDetailScreen now links to Wahl admin and each Gemeinde row opens Teamer
admin.

Backend: widen the wahl + files LT routes to AuthGuard(['authentik','team'])
for consistency with the other LT controllers. Rebrand web/index.html +
manifest from "kc_app" to "KC-App".

Verified against local Postgres with an isLeitungsteam team token: create
KC/Gemeinde/Wahl/Workshop, run Zuteilung, create Teamer + invite, resolve
an invite code. flutter analyze/test/build web green; backend npm test 56.

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

client/app/web/manifest.json

@@ -1,11 +1,11 @@
 {
-    "name": "kc_app",
-    "short_name": "kc_app",
+    "name": "KC-App",
+    "short_name": "KC-App",
     "start_url": ".",
     "display": "standalone",
     "background_color": "#0175C2",
     "theme_color": "#0175C2",
-    "description": "A new Flutter project.",
+    "description": "Konfi-Castle Event-, Wahl- und Kommunikationsplattform",
     "orientation": "portrait-primary",
     "prefer_related_applications": false,
     "icons": [