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>

web_src/js/index.ts

@@ -67,6 +67,7 @@ import {callInitFunctions} from './modules/init.ts';
 import {initRepoViewFileTree} from './features/repo-view-file-tree.ts';
 import {initActionsPermissionsForm} from './features/common-actions-permissions.ts';
 import {initGlobalShortcut} from './modules/shortcut.ts';
+import {initDevtest} from './modules/devtest.ts';
 
 const initStartTime = performance.now();
 const initPerformanceTracer = callInitFunctions([
@@ -160,6 +161,8 @@ const initPerformanceTracer = callInitFunctions([
 
   initRepoFileView,
   initActionsPermissionsForm,
+
+  initDevtest,
 ]);
 
 // it must be the last one, then the "querySelectorAll" only needs to be executed once for global init functions.