feat: add Sentry development dashboard

[musepy]

Summary

• add a standalone Flutter dashboard for reviewing Anycast app development issues from the current Sentry snapshot
• show 24H, 7D, and 30D metrics, signal coverage, event activity, and a prioritized issue queue
• support Impact, Recent, and New filters with links to Sentry issue pages
• add a widget test for period switching and issue filtering

Validation

• PASS: flutter analyze --no-pub --no-fatal-infos lib/sentry_dashboard.dart test/sentry_dashboard_test.dart
• PASS: flutter test --no-pub test/sentry_dashboard_test.dart
• PASS: git diff --cached --check
• BLOCKED ON BASELINE: flutter pub get --enforce-lockfile reports that upstream main would change 5 locked dependencies
• BLOCKED ON BASELINE: full analyze and test cannot resolve material_design_icons_flutter referenced by lib/widgets/bottom_nav_bar.dart; full analyze also reports the existing profilesSampleRate experimental warning

Safety

• read-only snapshot; no Sentry issue, project setting, release, signing, or store state was changed
• no credentials or tokens are included

The PR is draft because the repository-wide required checks are blocked by existing upstream dependency state.