Fix SharedPreferences key/type mismatches and upgrade safety

[timbortnik]

Summary

• Key mismatch: Kotlin now reads saved_* (user intent) instead of cached_* with
  fallback to legacy cached_* Float keys for upgrade compatibility
• Type mismatch: Correctly decodes home_widget doubles via getLong() +
  Double.fromBits() instead of getFloat()
• Race condition: Discards stale weather fetch if user changed location mid-request
• GPS persistence: _loadWeather now persists GPS coordinates to HomeWidget so background
  refresh (AlarmManager, WorkManager) has coordinates to use
• Migration: location_source → saved_location_source, removes redundant use_gps key,
  cleans both SharedPreferences and HomeWidget stores
• City clearing: saveLocation() removes stale city when called without one (e.g.
  switching from manual to GPS)
• Dead code: Removes unused getSavedLocationFromWidget()

Upgrade safety

• Dart migration in main.dart handles key renames (idempotent, runs on app open)
• Kotlin fallback in getLocation() bridges GPS users who haven't opened the app post-upgrade
• saveLocation() skips redundant writes when nothing changed

Note

This is a significant refactor for what were mostly latent issues (the old cached_* Float
round-trip worked in practice). The main real-world fixes are the 0,0 coordinate bug and the
race condition. Consider whether the complexity is warranted.

Test plan

• make analyze — no issues
• make test — all Dart + Kotlin tests pass
• Install on device with existing widget, verify background refresh works without opening
  app
• Open app, verify GPS coordinates persisted and widget refreshes
• Switch from manual to GPS, verify stale city cleared
• Fresh install, verify widget works after first app launch

🤖 Generated with Claude Code

[timbortnik]

Will revisit later