fix: stabilize BiliBili and NicoNico playback

[Priveetee]

Summary

• use backend-proxied NicoNico HLS manifests directly
• build a minimal BiliBili DASH manifest from supported streams
• add BiliBili DASH retry/fallback behavior and resume after player errors
• de-duplicate related videos before rendering

Validation

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