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/fr/settings.ts

@@ -351,7 +351,7 @@ export const settings = {
   themeStoreTitle: 'Boutique de thèmes',
   themeStoreSubmitText: "Vous avez créé votre propre thème ? Partagez-le avec la communauté — plus d'infos dans le dépôt de thèmes.",
   themeStoreSubmitLink: 'Ouvrir le dépôt de thèmes',
-  themeStoreNetworkNotice: 'Le Theme Store charge son catalogue et ses aperçus depuis des services externes (le CDN jsDelivr et GitHub). Aucune donnée personnelle n’est envoyée.',
+  themeStoreNetworkNotice: 'Le Theme Store charge son catalogue et ses aperçus depuis GitHub. Aucune donnée personnelle n’est envoyée.',
   themeStoreStatsNotice: 'Les dates de dernière modification sont actualisées une fois par jour (vers 04h17 UTC).',
   themeStoreSearchPlaceholder: 'Rechercher des thèmes…',
   themeStoreFilterMode: 'Filtrer par mode',