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/modules/devtest.ts

@@ -0,0 +1,20 @@
+import {showInfoToast, showWarningToast, showErrorToast} from './toast.ts';
+import type {Toast} from './toast.ts';
+import {registerGlobalInitFunc} from './observer.ts';
+
+type LevelMap = Record<string, (message: string) => Toast | null>;
+
+export function initDevtest() {
+  registerGlobalInitFunc('initDevtestPage', () => {
+    const els = document.querySelectorAll('.toast-test-button');
+    if (!els.length) return;
+    const levelMap: LevelMap = {info: showInfoToast, warning: showWarningToast, error: showErrorToast};
+    for (const el of els) {
+      el.addEventListener('click', () => {
+        const level = el.getAttribute('data-toast-level')!;
+        const message = el.getAttribute('data-toast-message')!;
+        levelMap[level](message);
+      });
+    }
+  });
+}