ci: add pull request quality checks

[skylartaylor]

What changed

• adds pull-request analysis and deterministic Flutter test coverage
• adds visual regression checks with readable failure summaries and uploaded difference artifacts
• builds an Android ARM64 debug APK on pull requests
• pins third-party actions and toolchains and enables dependency updates
• aligns the desktop release workflow with Flutter 3.44.1
• repairs test and build compatibility exposed by clean-runner verification

Why

Pull requests should carry reproducible code, visual, native-asset, and Android build evidence before merge.

Validation

• full Flutter test suite passed on Ubuntu
• analyzer, generated-file guard, and platform-specific checks passed
• six committed Ubuntu golden comparisons passed
• Android ARM64 debug APK built successfully
• native media-codec asset smoke test passed locally
• workflow syntax passed actionlint

GitHub Actions run