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>

stylelint.config.js

@@ -26,7 +26,7 @@ export default {
   ],
   overrides: [
     {
-      files: ['**/chroma/*', '**/codemirror/*', '**/standalone/*', '**/console.css', 'font_i18n.css'],
+      files: ['**/chroma/*', '**/codemirror/*', '**/console.css', 'font_i18n.css'],
       rules: {
         'scale-unlimited/declaration-strict-value': null,
       },