feat: automate version.g.dart generation via build_runner

Replace manual generate_version.dart script with a custom build_runner
builder that automatically generates lib/version.g.dart from pubspec.yaml
during the build process. This eliminates manual steps and ensures version
info stays in sync.

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

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

README.md

@@ -129,7 +129,9 @@ flutter run              # Default device
 
 ### Code Generation
 
-After modifying any `@freezed`, `@JsonSerializable`, or `@riverpod` annotated code:
+Run build_runner after modifying:
+- `@freezed`, `@JsonSerializable`, or `@riverpod` annotated code
+- `pubspec.yaml` version (auto-generates `lib/version.g.dart`)
 
 ```bash
 dart run build_runner build --delete-conflicting-outputs
@@ -138,6 +140,11 @@ dart run build_runner build --delete-conflicting-outputs
 dart run build_runner watch --delete-conflicting-outputs
 ```
 
+**Generated files**:
+- `*.freezed.dart` - Freezed immutable classes
+- `*.g.dart` - JSON serialization, Riverpod providers
+- `lib/version.g.dart` - App version from pubspec.yaml
+
 ## Deployment
 
 ### Web (Docker)