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>

pubspec.yaml

@@ -76,6 +76,7 @@ dev_dependencies:
   mocktail: ^1.0.4
 
   # Build tools
+  build: ^2.4.0
   yaml: ^3.1.0
 
 # For information on the generic Dart part of this file, see the