Cleanup: expired cache entries are now properly removed instead of piling up on disk.

Performance
- Cold-cache scans on link-heavy pages now up to 6x faster.
- Repo data fetching moved off the main thread into the MV3 service worker.
- Content script runs at document_start so cached results paint before the page finishes loading.

Privacy
- Required permissions narrowed: host_permissions is now https://api.github.com/* only (was <all_urls> in v1.0.0).
- Broad host access is now an optional permission, declared but not granted at install — reserved for a planned user-opt-in mode for private/intranet GitHub hosts.

UX
- Distinct chips for broken links, rate-limited responses, and other errors.
- Archive indicator on archived repositories.
- "Star us?" CTA in the popup.
- Toolbar icon style toggle (default star vs multicolor constellation).

Stability
- Eliminated multi-second stalls under heavy React-based hosts.