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/de/settings.ts

@@ -355,7 +355,7 @@ export const settings = {
   themeStoreTitle: 'Theme-Store',
   themeStoreSubmitText: 'Eigenes Theme gebaut? Teil es mit der Community — mehr Infos im Themes-Repository.',
   themeStoreSubmitLink: 'Themes-Repository öffnen',
-  themeStoreNetworkNotice: 'Der Theme-Store lädt Katalog und Vorschauen von externen Diensten (jsDelivr-CDN und GitHub). Es werden keine persönlichen Daten gesendet.',
+  themeStoreNetworkNotice: 'Der Theme-Store lädt Katalog und Vorschauen von GitHub. Es werden keine persönlichen Daten gesendet.',
   themeStoreStatsNotice: 'Die „Zuletzt geändert“-Daten werden einmal täglich aktualisiert (gegen 04:17 UTC).',
   themeStoreSearchPlaceholder: 'Themes suchen…',
   themeStoreFilterMode: 'Nach Modus filtern',