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>

tailwind.config.ts

@@ -1,11 +1,8 @@
 import {readFileSync} from 'node:fs';
-import {env} from 'node:process';
 import {parse} from 'postcss';
 import plugin from 'tailwindcss/plugin.js';
 import type {Config} from 'tailwindcss';
 
-const isProduction = env.NODE_ENV !== 'development';
-
 function extractRootVars(css: string) {
   const root = parse(css);
   const vars = new Set<string>();
@@ -29,8 +26,6 @@ export default {
   prefix: 'tw-',
   important: true, // the frameworks are mixed together, so tailwind needs to override other framework's styles
   content: [
-    isProduction && '!./templates/devtest/**/*',
-    isProduction && '!./web_src/js/standalone/devtest.ts',
     '!./templates/swagger/v1_json.tmpl',
     '!./templates/user/auth/oidc_wellknown.tmpl',
     '!**/*_test.go',