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>

templates/devtest/devtest-header.tmpl

@@ -1,8 +1,4 @@
 {{template "base/head" ctx.RootData}}
 <link rel="stylesheet" href="{{AssetURI "css/devtest.css"}}">
-<script>
-	// must make sure the jQuery is globally loaded
-	// ref: https://github.com/go-gitea/gitea/issues/35923
-	if (!window.jQuery) alert('jQuery is missing, user custom plugins may not work');
-</script>
+<div class="tw-hidden" data-global-init="initDevtestPage"></div>
 {{template "base/alert" .}}