feat: capture tab audio in an offscreen document

[Maitrog]

Summary

This PR replaces the legacy Window Mode capture path with a dedicated offscreen document that captures tab audio, moves capture lifecycle coordination into the background, starts capture directly from the popup, and hardens recovery and cleanup when captures end unexpectedly.

User-facing improvements

• Replaced Window Mode with tab capture that starts directly from the popup.
• Added a per-tab ON/OFF badge on the extension icon showing where capture is active.
• Spectrum visualization is now enabled by default; volume compensation is disabled by default.
• Improved capture reliability when:
  • the offscreen document is lost or killed (page mode is restored);
  • captures end while the popup is open (popup target is restored);
  • the offscreen document sits idle (closed after reconciliation);
  • captured tabs close or change quickly.
• Capture errors are owned by the background and shown as a clear error state in the popup; the ordinary equalizer state is shown when no capture runs.
• Kept toolkit-window tabs out of page-mode autostart.
• Updated localization files for 19 locales.

Architecture and build

• Added an offscreen document capture engine (offscreen.html, src/app/offscreen/) with the offscreen manifest permission and a dedicated build entry.
• Added background capture coordination:
  • capture coordinator owning start/stop, reconciliation, and idle offscreen cleanup;
  • a dedicated capture error controller;
  • tab mute and per-tab badge modules.
• Routed capture settings, keyboard shortcuts, and spectrum demand through the background; spectrum is relayed from the offscreen document through an explicit capture source and gated on the enabled setting.
• Removed the legacy Window Mode capture path (windowModeCoordinator, toolkit window controller).
• Separated persisted gain from transient equalizer state (persistedGain.ts).
• Expanded unit coverage for the capture coordinator, offscreen engine, popup capture controller, badge, mute, and messaging.

Verification

• npm test - 67 test files and 356 tests passed.
• npm run typecheck - passed.
• npm run lint - passed.
• Prettier verification against committed LF content - passed.
• npm run lint:locales - passed for 19 locales.
• npm run build - production build completed successfully.

Scope

• 19 commits.
• 98 changed files.
• 5,164 additions and 3,182 deletions.