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-footer.tmpl

@@ -1,3 +1 @@
-{{/* TODO: the devtest.js is isolated from index.js, so no module is shared and many index.js functions do not work in devtest.ts */}}
-<script type="module" src="{{AssetURI "js/devtest.js"}}"></script>
 {{template "base/footer" ctx.RootData}}