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>

services/webtheme/webtheme.go

@@ -5,6 +5,7 @@ package webtheme
 
 import (
 	"io/fs"
+	"net/url"
 	"os"
 	"path"
 	"regexp"
@@ -43,6 +44,10 @@ type ThemeMetaInfo struct {
 	ColorScheme    string
 }
 
+func (info *ThemeMetaInfo) PublicAssetURI() string {
+	return public.AssetURI("css/theme-" + url.PathEscape(info.InternalName) + ".css")
+}
+
 func (info *ThemeMetaInfo) GetDescription() string {
 	if info.ColorblindType == "red-green" {
 		return "Red-green colorblind friendly"