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/lib/screens/home_screen.dart

@@ -5,6 +5,7 @@ import '../main.dart';
 import 'admin_screen.dart';
 import 'chat_screen.dart';
 import 'files_screen.dart';
+import 'verantwortliche_register_screen.dart';
 import 'wahl_screen.dart';
 
 class HomeScreen extends StatelessWidget {
@@ -16,6 +17,10 @@ class HomeScreen extends StatelessWidget {
     final id = state.identity!;
     final kcId = id.kcId;
 
+    final needsVerantwRegistration = id.kind == SessionKind.user &&
+        !id.isLeitungsteam &&
+        id.memberships.isEmpty;
+
     final tiles = <Widget>[
       if (id.isLeitungsteam)
         _NavTile(
@@ -24,6 +29,13 @@ class HomeScreen extends StatelessWidget {
           subtitle: 'KCs, Gemeinden, Onboarding-Freigaben',
           onTap: () => _open(context, const AdminScreen()),
         ),
+      if (needsVerantwRegistration)
+        _NavTile(
+          icon: Icons.how_to_reg,
+          title: 'Als Verantwortliche/r registrieren',
+          subtitle: 'KC-Code eingeben, Gemeinde wählen, Freigabe abwarten',
+          onTap: () => _open(context, const VerantwortlicheRegisterScreen()),
+        ),
       if (id.kind == SessionKind.guest)
         _NavTile(
           icon: Icons.how_to_vote,