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/repo/render.go

@@ -42,7 +42,9 @@ func RenderFile(ctx *context.Context) {
 	rctx := renderhelper.NewRenderContextRepoFile(ctx, ctx.Repo.Repository, renderhelper.RepoFileOptions{
 		CurrentRefPath:  ctx.Repo.RefTypeNameSubURL(),
 		CurrentTreePath: path.Dir(ctx.Repo.TreePath),
-	}).WithRelativePath(ctx.Repo.TreePath).WithInStandalonePage(true)
+	}).WithRelativePath(ctx.Repo.TreePath).WithStandalonePage(markup.StandalonePageOptions{
+		CurrentWebTheme: ctx.TemplateContext.CurrentWebTheme(),
+	})
 	renderer, rendererInput, err := rctx.DetectMarkupRendererByReader(blobReader)
 	if err != nil {
 		http.Error(ctx.Resp, "Unable to find renderer", http.StatusBadRequest)