Fix Android QR scanner lifecycle and location fallback

[liuyunming521520-cell]

Summary

• make QR scanner CameraX bindings session-scoped and dispose only their own use cases
• surface camera-unavailable recovery instead of leaving the scanner unusable
• fall back to the Android system location provider when Fused Location returns no result or rejects updates
• show an accurate location-notes availability message

Validation

• :app:compileDebugKotlin --rerun-tasks
• :app:assembleDebug
• :app:lintDebug (the task completes; the repository has existing lint findings)
• manual device verification: QR scanning lifecycle and location notes both work after the change

Test limitation

• :app:testDebugUnitTest is currently blocked in this checkout by an existing test-runner classpath issue: compiled test classes are not found by the runner.