feat: LAN-first development with optional auth

- Default API URLs now use LAN IPs (192.168.86.149)
- Auth interceptor skips auth when using LAN endpoints
- Production builds override with --dart-define

Development: flutter run -d chrome (no auth needed on LAN)
Production:  flutter build web --dart-define=CORE_API_URL=https://api.schweitz.net

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/core/api/api_interceptors.dart

@@ -4,9 +4,12 @@ import 'package:dio/dio.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../auth/auth_provider.dart';
+import '../config/app_config.dart';
 import '../error/app_exception.dart';
 
 /// Adds authentication token to requests.
+///
+/// Skipped entirely when [AppConfig.requiresAuth] is false (LAN development).
 class AuthInterceptor extends Interceptor {
   AuthInterceptor(this._ref);
 
@@ -14,6 +17,12 @@ class AuthInterceptor extends Interceptor {
 
   @override
   void onRequest(RequestOptions options, RequestInterceptorHandler handler) {
+    // Skip auth for LAN development
+    if (!AppConfig.requiresAuth) {
+      handler.next(options);
+      return;
+    }
+
     final authState = _ref.read(authNotifierProvider);
 
     authState.whenData((auth) {
@@ -27,6 +36,12 @@ class AuthInterceptor extends Interceptor {
 
   @override
   void onError(DioException err, ErrorInterceptorHandler handler) {
+    // Skip auth error handling for LAN development
+    if (!AppConfig.requiresAuth) {
+      handler.next(err);
+      return;
+    }
+
     if (err.response?.statusCode == 401) {
       // Token expired - trigger re-authentication
       _ref.read(authNotifierProvider.notifier).signOut();