feat: start YouTube transcriptions from Chrome

[MyButtermilk]

Summary

• add a narrow Manifest V3 Chrome extension with an inline YouTube action and toolbar fallback
• register and strictly validate the versioned scriber://youtube/transcribe desktop handoff
• forward secondary Windows launches to the already-running tray instance without exposing the backend token
• consume the one-shot request in the authenticated WebView and reuse the existing captions-first YouTube job
• add the public Chrome Web Store privacy disclosure and document installation, architecture, security boundaries, and focused gates

Validation

• node --test browser-extension\chrome\tests\shared.test.cjs (3 passed)
• cd Frontend && npm run test:lib (98 passed)
• cd Frontend && npm run check
• cd Frontend && npm run lint
• cd Frontend && npm run build:webview
• scripts\project-python.cmd -m pytest tests\api\test_youtube_routes.py -q (15 passed)
• cargo fmt -- --check
• cargo metadata --locked --no-deps --format-version 1
• loaded the unpacked extension in an isolated Chrome-for-Testing profile and verified the button on a live YouTube watch page

Native test note

The focused Rust unit tests could not reach the crate on this workstation because its Visual Studio installation lacks the Windows SDK headers/libraries (windows.h, kernel32.lib). The PR's Windows CI runner should provide the authoritative native build/test result.

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Preserve every queued browser handoff

When two extension launches reach a starting primary instance before the WebView navigation listener is ready, this loop drains both requests and calls queue_youtube_deep_link_navigation for each, but PendingNavigationState::queue stores only one pending value. The emitted events have no listener yet, so each iteration overwrites the previous request and only the final video is transcribed; retain a queue or wait for acknowledgement before advancing to the next request.

AGENTS.md reference: AGENTS.md:L346-L350

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