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>

lib/version.g.dart

@@ -1,5 +1,5 @@
 // GENERATED FILE - DO NOT EDIT
-// Generated by: dart run tool/generate_version.dart
+// Generated by build_runner from pubspec.yaml
 
 /// Application version information from pubspec.yaml
 class AppVersion {