Plan: playlist support feature

[rthaut]

Summary

• Adds a docs-only playlist feature implementation plan at docs/plans/playlist-feature-plan.md, covering issues Scrollable playlist navigation with links inside popout player window? ~ 2026 Youtube webplayer UI/UX debacle❗ #454, Ability to add video(s) to an existing popout playlist. #418, Playlist Shuffle Support #401, and Support For Temporary Queue Playlist #394.
• Documents current repo touchpoints, YouTube embed/IFrame Player API/Data API constraints, browser extension constraints, a proposed extension-owned playlist session architecture, per-issue implementation paths, recommended sequencing, testing strategy, and risks (including brittle YouTube DOM dependencies).
• Includes GitHub issue comment drafts for each related feature request.

Recreated on a branch based on current master; supersedes #466. The intervening master changes (dependency bumps including MUI 9, workflow updates, Renovate config removal) do not affect the plan's referenced files or substance.

Validation

• npx markdownlint-cli2 docs/plans/**/*.md
• npx prettier --check docs/plans/playlist-feature-plan.md