Unify re:Watch notifications and refresh controls

[EdinUser]

Summary

• replace the legacy player-attached active notice with the shared branded re:Watch toast
• queue onboarding and AI-label announcements so only one is active across YouTube tabs, and open the normal browser action popup from onboarding
• make System Default theme consistent between the Firefox popup and feed
• start the sidebar expanded and clarify Reload videos versus Refresh
• update release notes, user and maintainer documentation, and regenerated guide screenshots

Validation

• full Jest suite: 325 passed, 9 skipped
• lint
• documentation safety and MkDocs build
• Chrome packaged extension suite: 15 passed, 1 live canary skipped
• Firefox packaged extension regression suite passed