chore: remove tracked generated artifacts

[jjoonleo]

Summary

• Remove generated coverage reports, iOS build cache files, root Firebase hosting cache files, and Widgetbook Firebase hosting output from git tracking using git rm --cached.
• Add narrow ignore rules for /coverage/, /ios/build/, root /.firebase/, and widgetbook/.firebase/.
• Keep required source/config files tracked: .firebaserc, firebase.json, widgetbook/.firebaserc, and widgetbook/firebase.json.

Closes #538

Verification

• Before cleanup: git ls-files coverage ios/build .firebase widgetbook/.firebase returned 350 tracked generated files: .firebase 2, coverage 209, ios/build 85, widgetbook/.firebase 54.
• After cleanup: git ls-files -- coverage ios/build .firebase widgetbook/.firebase returns 0.
• git check-ignore -v coverage/lcov.info .firebase/hosting.d2Vi.cache widgetbook/.firebase/hosting.YnVpbGQvd2Vi.cache confirms the new ignore rules cover regenerated artifacts.
• find ios/build -type f -print -quit | xargs -r git check-ignore -v -- confirms the ios/build rule covers an existing generated iOS cache file.
• git ls-files .firebaserc firebase.json widgetbook/.firebaserc widgetbook/firebase.json confirms Firebase source/config files remain tracked.
• git diff --check passed.
• dart run build_runner build --delete-conflicting-outputs completed locally to regenerate ignored Dart outputs needed by analyzer.
• flutter analyze passed after regeneration. The first analyzer run failed on missing generated files in the fresh worktree, before any source-code issue was found.