feat(youtube): browser extension skeleton for YouTube and YouTube Music (#136)

[rowkav09]

Part of #136, slice 2. Adds extension/youtube/, a Manifest V3 extension for Chrome and Edge. It's separate from the app, and nothing in the app changes.

• Permissions: storage, http://127.0.0.1/*, and content scripts on www.youtube.com and music.youtube.com. No tabs, history, cookies or other sites.
• reader.js: reads only the player on the page: video ID from the URL, title, channel and artwork from the page's media session, position, duration, paused and live. Every YouTube-specific selector is in this one file. Shorts and ads return "skip", and non-watch pages return nothing.
• content.js: reports while playing (every 10 s and on play/pause/end), reports a pause once, and says "stopped" on ads, Shorts, leaving the page or closing the tab.
• background.js: adds a per-tab ID and posts to 127.0.0.1:<port>/bridge/youtube with the pairing code. Nothing is sent until paired.
• Options page: paste the pairing code, set the port, unpair. Plain layout, blue and orange only, keyboard focus visible.

Tests run the real reader.js against fixture pages (normal, paused, Music, ad, Shorts, live, search page). They check the manifest's permissions, and that the events it builds pass the app's bridge schema from #395.

Not yet: the app route and pairing code (#405 plus the next PR), and zipping it in CI (release lane). Testers can load the folder unpacked for now.

Checks

• Tests: reader fixtures, manifest permissions, schema compatibility
• No secrets committed
• Scoped to feat: add YouTube playback as an optional provider plugin #136

[rowkav09]

/mira resume

[github-actions]

/mira pause

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

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[rowkav09]

No Mira review came in after /mira resume (Mira is still paused). Rebased, tests pass and every check is green. I reviewed it myself: minimal permissions, selectors isolated in reader.js, nothing sent until paired, and it doesn't touch the app. Merging with an explicit squash message.