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/ui/preferences.rs

@@ -413,6 +413,44 @@ fn build_updates_page(settings: &gio::Settings) -> adw::PreferencesPage {
 
     page.add(&security_group);
 
+    // Catalog Enrichment group
+    let enrichment_group = adw::PreferencesGroup::builder()
+        .title(&i18n("Catalog Enrichment"))
+        .description(&i18n("Fetch GitHub metadata (stars, version, downloads) for catalog apps"))
+        .build();
+
+    let auto_enrich_row = adw::SwitchRow::builder()
+        .title(&i18n("Auto-enrich catalog apps"))
+        .subtitle(&i18n("Fetch metadata from GitHub in the background"))
+        .active(settings.boolean("catalog-auto-enrich"))
+        .build();
+    let settings_enrich = settings.clone();
+    auto_enrich_row.connect_active_notify(move |row| {
+        settings_enrich.set_boolean("catalog-auto-enrich", row.is_active()).ok();
+    });
+    enrichment_group.add(&auto_enrich_row);
+
+    let token_row = adw::PasswordEntryRow::builder()
+        .title(&i18n("GitHub token"))
+        .build();
+    let current_token = settings.string("github-token");
+    if !current_token.is_empty() {
+        token_row.set_text(&current_token);
+    }
+    let settings_token = settings.clone();
+    token_row.connect_changed(move |row| {
+        settings_token.set_string("github-token", &row.text()).ok();
+    });
+    enrichment_group.add(&token_row);
+
+    let token_hint = adw::ActionRow::builder()
+        .title(&i18n("Optional - increases rate limit from 60 to 5,000 requests per hour"))
+        .css_classes(["dim-label"])
+        .build();
+    enrichment_group.add(&token_hint);
+
+    page.add(&enrichment_group);
+
     page
 }