feat: add podcasts and saved collections

[Priveetee]

Summary

• Add channel sorting and YouTube podcast browsing.
• Move Favorites and Watch Later to dedicated endpoints and pages.
• Stream downloader artifacts directly with cancel and clear actions.
• Add responsive search suggestions/history and timestamp seeking in descriptions/comments.
• Refine player language defaults, cinema layout, NicoNico playback, and admin settings.

Validation

• bun run check
• bun run build
• bun run knip
• bun run sherif
• git diff --check