feat(backend): Wahl-Phasen, Verantwortliche-Invites, Auth fixes

- New VerantwortlicheInvite model: LT-issued invites so a person can
  register as Gemeinde Verantwortliche(r) for a specific Gemeinde,
  skipping the self-registration approval step.
- Wahl/Workshop/Teilnehmer gain phase support (phasenAnzahl,
  beschreibung), mirroring the WP plugin's multi-phase elections.
  Teilnehmer unique constraint now scoped per phase.
- Auth: team login + guest auth adjustments, spec coverage.
- sync.service.ts: register VerantwortlicheInvite as a synced model.
- wahl.service.ts: submitTeilnehmer updated for the new phase-scoped
  unique key.
- client: login/home screen rework, new theme.dart, FCM web tweaks.
- .gitignore: ignore .DS_Store.

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

client/app/lib/theme.dart

@@ -0,0 +1,99 @@
+import 'package:flutter/material.dart';
+
+/// Brand palette lifted from konfi-castle.com (Kubio theme CSS custom
+/// properties: --kubio-color-1..6) so the app's look leans on the same
+/// identity as the marketing site instead of Flutter's default Material
+/// purple.
+abstract final class KcColors {
+  static const blue = Color(0xFF2F7CFF); // --kubio-color-1
+  static const orange = Color(0xFFF17C20); // --kubio-color-2
+  static const teal = Color(0xFF4EBA9A); // --kubio-color-3
+  static const slate = Color(0xFF69768B); // --kubio-color-4
+  static const navy = Color(0xFF2B2D42); // --kubio-color-6 (headings/text)
+  static const surface = Color(0xFFF7F9FC);
+}
+
+ThemeData buildKcTheme() {
+  final colorScheme = ColorScheme.fromSeed(
+    seedColor: KcColors.blue,
+    brightness: Brightness.light,
+  ).copyWith(
+    primary: KcColors.blue,
+    secondary: KcColors.orange,
+    tertiary: KcColors.teal,
+    surface: KcColors.surface,
+    onSurface: KcColors.navy,
+  );
+
+  return ThemeData(
+    useMaterial3: true,
+    colorScheme: colorScheme,
+    scaffoldBackgroundColor: KcColors.surface,
+    appBarTheme: AppBarTheme(
+      backgroundColor: KcColors.surface,
+      foregroundColor: KcColors.navy,
+      elevation: 0,
+      centerTitle: false,
+      titleTextStyle: const TextStyle(
+        color: KcColors.navy,
+        fontSize: 20,
+        fontWeight: FontWeight.w700,
+      ),
+    ),
+    cardTheme: CardThemeData(
+      elevation: 0,
+      color: Colors.white,
+      surfaceTintColor: Colors.transparent,
+      shape: RoundedRectangleBorder(
+        borderRadius: BorderRadius.circular(20),
+        side: BorderSide(color: KcColors.navy.withValues(alpha: 0.06)),
+      ),
+    ),
+    inputDecorationTheme: InputDecorationTheme(
+      filled: true,
+      fillColor: KcColors.surface,
+      border: OutlineInputBorder(
+        borderRadius: BorderRadius.circular(12),
+        borderSide: BorderSide.none,
+      ),
+      focusedBorder: OutlineInputBorder(
+        borderRadius: BorderRadius.circular(12),
+        borderSide: const BorderSide(color: KcColors.blue, width: 1.5),
+      ),
+      contentPadding: const EdgeInsets.symmetric(horizontal: 16, vertical: 14),
+    ),
+    filledButtonTheme: FilledButtonThemeData(
+      style: FilledButton.styleFrom(
+        backgroundColor: KcColors.blue,
+        foregroundColor: Colors.white,
+        minimumSize: const Size.fromHeight(50),
+        shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(12)),
+        textStyle: const TextStyle(fontWeight: FontWeight.w600, fontSize: 15),
+      ),
+    ),
+    outlinedButtonTheme: OutlinedButtonThemeData(
+      style: OutlinedButton.styleFrom(
+        foregroundColor: KcColors.blue,
+        side: const BorderSide(color: KcColors.blue),
+        minimumSize: const Size.fromHeight(46),
+        shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(12)),
+      ),
+    ),
+    segmentedButtonTheme: SegmentedButtonThemeData(
+      style: SegmentedButton.styleFrom(
+        selectedBackgroundColor: KcColors.blue,
+        selectedForegroundColor: Colors.white,
+        foregroundColor: KcColors.navy,
+        side: BorderSide(color: KcColors.navy.withValues(alpha: 0.15)),
+      ),
+    ),
+    textTheme: const TextTheme(
+      titleLarge: TextStyle(color: KcColors.navy, fontWeight: FontWeight.w700),
+      titleMedium: TextStyle(color: KcColors.navy, fontWeight: FontWeight.w600),
+      bodyMedium: TextStyle(color: KcColors.navy),
+      bodySmall: TextStyle(color: KcColors.slate),
+      labelMedium: TextStyle(color: KcColors.slate),
+    ),
+    dividerTheme: DividerThemeData(color: KcColors.navy.withValues(alpha: 0.08)),
+  );
+}