Fix Android setup and clean up Dart code

[YspritanHyzygy]

Summary

• Let Android release builds configure without a local key.properties file by falling back to debug signing.
• Correct the Google Play resource URL and keep stale cached or remote resource data from restoring the old application ID.
• Consolidate generated Android/build entries in .gitignore.
• Clean up Dart analyzer issues and guard dialog navigation after asynchronous language changes.
• Add a regression test for the Google Play application ID.

These maintenance changes were extracted from #36 so the Simplified Chinese localization can be reviewed independently.

Validation

• Parsed assets/additional_resources.json with jq.
• Verified the Play Store resource matches Android application ID page.puzzak.paios.
• Confirmed the old Play Store application ID and renamed Dart identifiers are absent.
• git diff --check upstream/main
• git merge-tree --write-tree codex/simplified-chinese-localization codex/maintenance-fixes (clean merge)

flutter analyze and flutter test were not rerun in the current environment because no Flutter SDK is installed. The original combined commit reported both checks passing; this split retains the same production changes and separates the regression tests by topic.