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>

tools/test-e2e.sh

@@ -40,6 +40,13 @@ EVENT_SOURCE_UPDATE_TIME = 500ms
 [log]
 MODE = console
 LEVEL = Warn
+
+[markup.test-external]
+ENABLED = true
+FILE_EXTENSIONS = .external
+RENDER_COMMAND = cat
+IS_INPUT_FILE = false
+RENDER_CONTENT_MODE = iframe
 EOF
 
 export GITEA_WORK_DIR="$WORK_DIR"