refactor: add AuthGuard component and fix API client empty response handling

- Add reusable AuthGuard component for protected routes
- Move auth logic from index.tsx to (tabs)/_layout.tsx via AuthGuard
- Fix ApiClient to handle empty responses (204 No Content)
- Use useFocusEffect in chat.tsx to load messages after auth is ready
- Extract getDateKey() helper function in calendar.tsx

CLAUDE.md

@@ -81,6 +81,7 @@ src/
 │   └── note/
 │       └── [id].tsx            # Note editor for event (dynamic route)
 ├── components/
+│   ├── AuthGuard.tsx           # Auth wrapper: loads user, shows loading, redirects if unauthenticated
 │   ├── BaseBackground.tsx      # Common screen wrapper (themed)
 │   ├── BaseButton.tsx          # Reusable button component (themed, supports children)
 │   ├── Header.tsx              # Header component (themed)
@@ -97,7 +98,7 @@ src/
 │   └── logger.ts               # react-native-logs config (apiLogger, storeLogger)
 ├── services/
 │   ├── index.ts                # Re-exports all services
-│   ├── ApiClient.ts            # HTTP client with X-User-Id header injection, request logging
+│   ├── ApiClient.ts            # HTTP client with X-User-Id header injection, request logging, handles empty responses (204)
 │   ├── AuthService.ts          # login(), register(), logout() - calls API and updates AuthStore
 │   ├── EventService.ts         # getAll(), getById(), getByDateRange(), create(), update(), delete()
 │   └── ChatService.ts          # sendMessage(), confirmEvent(), rejectEvent(), getConversations(), getConversation()
@@ -112,6 +113,13 @@ src/
 
 **Routing:** Tab-based navigation with Chat, Calendar, and Settings as main screens. Auth screens (login, register) outside tabs. Dynamic routes for event detail and note editing.
 
+**Authentication Flow:**
+- `AuthGuard` component wraps the tab layout in `(tabs)/_layout.tsx`
+- On app start, `AuthGuard` calls `loadStoredUser()` and shows loading indicator
+- If not authenticated, redirects to `/login`
+- `index.tsx` simply redirects to `/(tabs)/chat` - AuthGuard handles the rest
+- This pattern handles Expo Router's navigation state caching (avoids race conditions)
+
 ### Theme System
 
 The app supports multiple themes (light/dark) via a reactive Zustand store.
@@ -412,12 +420,14 @@ NODE_ENV=development        # development = pretty logs, production = JSON
 - **Authentication fully implemented:**
   - `AuthStore`: Manages user state with expo-secure-store (native) / localStorage (web)
   - `AuthService`: login(), register(), logout() - calls backend API
-  - `ApiClient`: Automatically injects X-User-Id header for authenticated requests
+  - `ApiClient`: Automatically injects X-User-Id header for authenticated requests, handles empty responses (204)
+  - `AuthGuard`: Reusable component that wraps protected routes - loads user, shows loading, redirects if unauthenticated
   - Login screen: Supports email OR userName login
   - Register screen: Email validation, checks for existing email/userName
   - `AuthButton`: Reusable button component with themed shadow
   - `Header`: Themed header component (logout moved to Settings)
-  - `index.tsx`: Auth redirect - checks stored user on app start
+  - `(tabs)/_layout.tsx`: Wraps tabs with AuthGuard for protected access
+  - `index.tsx`: Simple redirect to chat (AuthGuard handles auth)
 - **Theme system fully implemented:**
   - `ThemeStore`: Zustand store with theme state and setTheme()
   - `Themes.tsx`: THEMES object with defaultLight/defaultDark variants
@@ -438,7 +448,7 @@ NODE_ENV=development        # development = pretty logs, production = JSON
   - Arrow navigation between proposals with "Event X von Y" counter
   - Each proposal individually confirmable/rejectable
   - **Typing indicator**: Animated dots (. .. ...) shown after 500ms delay while waiting for AI response
-  - Messages persisted to database via ChatService and loaded on mount
+  - Messages persisted to database via ChatService, loaded via `useFocusEffect` when screen gains focus
   - Tracks conversationId for message continuity across sessions
   - ChatStore with addMessages() for bulk loading, chatMessageToMessageData() helper
   - KeyboardAvoidingView for proper keyboard handling (iOS padding, Android height)