fix: Use internet bitrate for mobile playback

[Charlie284]

Summary

• Select the home bitrate for Wi-Fi and Ethernet playback.
• Select the internet bitrate for mobile and existing non-home connectivity states.
• Add regression coverage for Auto, a fixed 10 Mbps mobile limit, and offline semantics.

The playback creation path always read maxHomeBitrate, even when connectivityStatusProvider reported mobile data. The selected connection-specific setting now feeds the same quality options used to build the Jellyfin playback request and subsequent reloads.

Fixes #1082

Testing

• dart format test/playback_bitrate_helper_test.dart (no changes)
• flutter analyze --no-pub with Flutter 3.35.7 (no issues)
• flutter test --no-pub test/playback_bitrate_helper_test.dart test/pip_manager_test.dart (12 passed)
• flutter test --no-pub (12 passed, 1 pre-existing failure in test/widget_test.dart; it mounts Main without a ProviderScope and still expects the removed counter UI)