feat: Update Flutter version handling in CI and adjust pubspec files for compatibility

apps/mobile/pubspec.lock

@@ -1530,4 +1530,4 @@ packages:
     version: "2.2.3"
 sdks:
   dart: ">=3.10.3 <4.0.0"
-  flutter: ">=3.38.4"
+  flutter: ">=3.38.4 <4.0.0"

apps/mobile/pubspec.yaml

@@ -3,6 +3,7 @@ publish_to: 'none'
 description: "A sample project using melos and modular scaffold."
 environment:
   sdk: '>=3.10.0 <4.0.0'
+  flutter: '>=3.38.0 <4.0.0'
 workspace:
   - packages/design_system
   - packages/core