feat(01-01): Create secure Supabase configuration system

- Created .env template with SUPABASE_URL and SUPABASE_ANON_KEY placeholders
- Added .env to .gitignore to prevent committing secrets
- Created SupabaseConstants class with secure environment loading
- Added validation to ensure required environment variables are set
- Created core/constants directory structure

lib/core/constants/supabase_constants.dart

@@ -0,0 +1,25 @@
+import 'package:flutter_dotenv/flutter_dotenv.dart';
+
+/// Supabase configuration constants
+/// Environment variables are loaded securely from .env file
+class SupabaseConstants {
+  SupabaseConstants._();
+
+  static late final String supabaseUrl;
+  static late final String supabaseAnonKey;
+
+  /// Initialize Supabase constants from environment variables
+  static Future<void> initialize() async {
+    await dotenv.load(fileName: '.env');
+    
+    supabaseUrl = dotenv.env['SUPABASE_URL'] ?? '';
+    supabaseAnonKey = dotenv.env['SUPABASE_ANON_KEY'] ?? '';
+    
+    if (supabaseUrl.isEmpty || supabaseAnonKey.isEmpty) {
+      throw Exception(
+        'SUPABASE_URL and SUPABASE_ANON_KEY must be set in .env file\n'
+        'Get these values from Supabase Dashboard → Settings → API',
+      );
+    }
+  }
+}