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

apps/client/src/services/ApiClient.ts

@@ -55,7 +55,12 @@ async function request<T>(
     }
 
     apiLogger.debug(`${method} ${endpoint} - ${response.status} (${duration}ms)`);
-    return response.json();
+
+    const text = await response.text();
+    if (!text) {
+      return undefined as T;
+    }
+    return JSON.parse(text);
   } catch (error) {
     const duration = Math.round(performance.now() - start);
     apiLogger.error(`${method} ${endpoint} failed (${duration}ms): ${error}`);