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/css/devtest.css

@@ -0,0 +1,16 @@
+.button-sample-groups {
+  margin: 0; padding: 0;
+}
+
+.button-sample-groups .sample-group {
+  list-style: none; margin: 0; padding: 0;
+}
+
+.button-sample-groups .sample-group .ui.button {
+  margin-bottom: 5px;
+}
+
+h1, h2 {
+  margin: 0;
+  padding: 10px 0;
+}