feat(config): add ENV shorthand for environment override

Add support for --dart-define=ENV=dev as a convenience shorthand
alongside the existing ENVIRONMENT override. Maps 'dev'/'local' to
local environment and 'prod'/'production' to production.

Also remove macos Flutter config files from tracking (already in gitignore).

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

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

lib/config/environment_config.dart

···
   static const String _flavor = String.fromEnvironment('FLUTTER_FLAVOR');
 
   /// Explicit environment override via --dart-define=ENVIRONMENT=local
+  /// Also supports --dart-define=ENV=dev for convenience
   static const String _envOverride = String.fromEnvironment('ENVIRONMENT');
+  static const String _envShorthand = String.fromEnvironment('ENV');
 
   /// Get current environment based on build configuration
   ///
···
       switch (_envOverride) {
         case 'local':
           return local;
+        case 'production':
+          return production;
+      }
+    }
+
+    // Priority 1b: Shorthand ENV override (dev -> local, prod -> production)
+    if (_envShorthand.isNotEmpty) {
+      switch (_envShorthand) {
+        case 'dev':
+        case 'local':
+          return local;
+        case 'prod':
         case 'production':
           return production;
       }

macos/Flutter/Flutter-Debug.xcconfig

···
-#include "ephemeral/Flutter-Generated.xcconfig"

macos/Flutter/Flutter-Release.xcconfig

···
-#include "ephemeral/Flutter-Generated.xcconfig"