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/nb/settings.ts

@@ -354,7 +354,7 @@ export const settings = {
   themeStoreTitle: 'Temabutikk',
   themeStoreSubmitText: 'Laget ditt eget tema? Del det med fellesskapet — mer info i tema-repoet.',
   themeStoreSubmitLink: 'Åpne tema-repoet',
-  themeStoreNetworkNotice: 'Theme Store laster katalogen og forhåndsvisninger fra eksterne tjenester (jsDelivr-CDN og GitHub). Ingen personopplysninger sendes.',
+  themeStoreNetworkNotice: 'Theme Store laster katalogen og forhåndsvisninger fra GitHub. Ingen personopplysninger sendes.',
   themeStoreStatsNotice: 'Sist endret-datoene oppdateres én gang om dagen (rundt 04:17 UTC).',
   themeStoreSearchPlaceholder: 'Søk i temaer…',
   themeStoreFilterMode: 'Filtrer etter modus',