feat: update API base URL handling and improve Caddy configuration for Flutter app

flutter/lib/core/api/api_client.dart

@@ -1,8 +1,8 @@
 import 'package:http/http.dart' as http;
 
-/// Platform-neutral HTTP client wrapping the internal API base URL.
-/// Base URL is read from the FLUTTER_API_URL_INTERNAL environment variable
-/// (set by Docker) or falls back to localhost for local development.
+/// Platform-neutral HTTP client.
+/// API base URL is injected at build time via --dart-define=API_BASE_URL.
+/// Default is same-origin '/api' to avoid mixed-content on HTTPS sites.
 class ApiClient {
   final String baseUrl;
   final http.Client _client;
@@ -10,7 +10,7 @@ class ApiClient {
   ApiClient({http.Client? client})
       : baseUrl = const String.fromEnvironment(
           'API_BASE_URL',
-          defaultValue: 'http://localhost:8080',
+                    defaultValue: '/api',
         ),
         _client = client ?? http.Client();