Merge some standalone Vite entries into index.js (#37085)

Keep `swagger` and `external-render-helper` as a standalone entries for
external render.

- Move `devtest.ts` to `modules/` as init functions
- Make external renders correctly load its helper JS and Gitea's current theme
- Make external render iframe inherit Gitea's iframe's background color to avoid flicker
- Add e2e tests for external render and OpenAPI iframe

---------

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/web.go

@@ -260,7 +260,7 @@ func Routes() *web.Router {
 	routes.BeforeRouting(chi_middleware.GetHead)
 
 	routes.Head("/", misc.DummyOK) // for health check - doesn't need to be passed through gzip handler
-	routes.Methods("GET, HEAD, OPTIONS", "/assets/*", routing.MarkLogLevelTrace, optionsCorsHandler(), public.FileHandlerFunc())
+	routes.Methods("GET, HEAD, OPTIONS", "/assets/*", routing.MarkLogLevelTrace, public.AssetsCors(), public.FileHandlerFunc())
 	routes.Methods("GET, HEAD", "/avatars/*", avatarStorageHandler(setting.Avatar.Storage, "avatars", storage.Avatars))
 	routes.Methods("GET, HEAD", "/repo-avatars/*", avatarStorageHandler(setting.RepoAvatar.Storage, "repo-avatars", storage.RepoAvatars))
 	routes.Methods("GET, HEAD", "/apple-touch-icon.png", misc.StaticRedirect("/assets/img/apple-touch-icon.png"))