fix(build): ensure fresh Flutter build on each deploy

- Add flutter clean before build to prevent stale cached artifacts
- Add VERSION build arg for explicit cache busting
- Reorder build steps: clean → pub get → build_runner → health.json → build
- Replace deprecated dart:html with package:web in iframe_view_web.dart
- Add lint ignore to generate_health_json.dart

Fixes issue where Docker layer caching kept old main.dart.js
while regenerating health.json with new version number.

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

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

tool/generate_health_json.dart

@@ -2,6 +2,8 @@
 // Generates web/health.json from pubspec.yaml
 // Run: dart run tool/generate_health_json.dart
 
+// ignore_for_file: avoid_print
+
 import 'dart:convert';
 import 'dart:io';