feat(ui): add singer selector dialog with engine grouping, search, and favorites

[yjzxkxdn]

Summary

This PR redesigns singer selection into a dedicated dialog and separates singers by synthesis engine for easier browsing.

What changed

• Added SingerSelectorDialog and SingerSelectorViewModel.
• Opened singer selector from track header singer button while keeping existing confirm/apply workflow.
• Grouped singers by engine: UTAU / ENUNU / DiffSinger / VOICEVOX / Vogen.
• Added search box (filter by singer name/id).
• Added favorite toggle and "favorites only" filter.
• Improved image fallback behavior:
  • If portrait is missing, avatar is used for both list card and right preview.
• UI polish:
  • Tuned card/list layout and right preview panel ratio.
  • Removed default blue/gray hover/focus artifacts in selector cards and favorite button.
  • Reduced corner radius for a more square visual style.

[yqzhishen]