[Feature]: Make YouTube web playback SABR-only

[Priveetee]

Summary

TypeType currently exposes two YouTube playback modes in the web app: Classic and SABR.

SABR is now the supported playback path, but removing Classic immediately would still be premature. This issue tracks the final migration after SABR has received enough beta validation.

Scope

• remove the Classic/SABR toggle from the navbar and settings;
• remove the playback transition modal;
• migrate or ignore stored Classic playback preferences;
• always use the YouTube SABR extraction and bootstrap endpoints;
• always use the TypeType MSE player for YouTube playback;
• cover watch pages, embeds and autoplay preloading.

NicoNico and BiliBili playback must remain unchanged.

The Server classic extraction contracts must also remain available for other providers and API compatibility. This only removes Classic as a user-facing YouTube web playback mode.

Rollout

This should remain in the roadmap backlog until VOD, livestream, seek, codec switching, audio tracks and embed playback have received enough beta testing.

The change must be deployed to beta first and remain there before any production rollout.

Acceptance criteria

• the web app no longer exposes a Classic playback option;
• old local storage values cannot restore Classic YouTube playback;
• the Frontend no longer requests /streams/youtube/legacy;
• YouTube watch pages and embeds use SABR/MSE;
• NicoNico and BiliBili playback remain unchanged;
• no classic Server endpoint is removed.