fix(themes): serve the Theme Store from GitHub raw, not jsDelivr (#1051)

Theme assets (registry.json, theme CSS, thumbnails) were served from the
jsDelivr CDN's mutable @main edge, which caches up to 12h. The registry is
purged on every themes push so updates were offered promptly — but the theme
CSS could still be served stale, so an update stored pre-update CSS under the
new version label and no further update was offered to correct it (a freshly
animated theme would install without its @keyframes).

Fetch everything from GitHub raw (permissive CORS, ~5-min server cache), which
is always current. jsDelivr is dropped entirely: our request volume does not
need a CDN, and its only real upside here (high-traffic edge serving) does not
apply, while its staleness actively caused the bug.

Rename cdnUrl -> assetUrl to reflect the source.

src/locales/en/settings.ts

@@ -399,7 +399,7 @@ export const settings = {
   themeStoreTitle: 'Theme Store',
   themeStoreSubmitText: 'Made your own theme? Share it with the community — more info in the themes repository.',
   themeStoreSubmitLink: 'Open the themes repository',
-  themeStoreNetworkNotice: 'The Theme Store loads its catalogue and previews from external services (the jsDelivr CDN and GitHub). No personal data is sent.',
+  themeStoreNetworkNotice: 'The Theme Store loads its catalogue and previews from GitHub. No personal data is sent.',
   themeStoreStatsNotice: 'Last-changed dates refresh once a day (around 04:17 UTC).',
   themeStoreSearchPlaceholder: 'Search themes…',
   themeStoreFilterMode: 'Filter by mode',