Add GitHub metadata enrichment for catalog apps

Enrich catalog apps with GitHub API data (stars, version, downloads,
release date) via two strategies: background drip for repo-level info
and on-demand fetch when opening a detail page.

- Add github_enrichment module with API calls, asset filtering, and
  architecture auto-detection for AppImage downloads
- DB migrations v14/v15 for GitHub metadata and release asset columns
- Extract github_owner/repo from feed links during catalog sync
- Display colored stat cards (stars, version, downloads, released) on
  detail pages with on-demand enrichment
- Show stars and version on browse tiles and featured carousel cards
- Replace install button with SplitButton dropdown when multiple arch
  assets available, preferring detected architecture
- Disable install button until enrichment completes to prevent stale
  AppImageHub URL downloads
- Keep enrichment banner visible on catalog page until truly complete,
  showing paused state when rate-limited
- Add GitHub token and auto-enrich toggle to preferences

src/core/mod.rs

@@ -4,6 +4,7 @@ pub mod backup;
 pub mod catalog;
 pub mod database;
 pub mod discovery;
+pub mod github_enrichment;
 pub mod duplicates;
 pub mod footprint;
 pub mod fuse;