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>

routers/web/misc/swagger.go

@@ -6,15 +6,9 @@ package misc
 import (
 	"net/http"
 
-	"code.gitea.io/gitea/modules/templates"
 	"code.gitea.io/gitea/services/context"
 )
 
-// tplSwagger swagger page template
-const tplSwagger templates.TplName = "swagger/ui"
-
-// Swagger render swagger-ui page with v1 json
 func Swagger(ctx *context.Context) {
-	ctx.Data["APIJSONVersion"] = "v1"
-	ctx.HTML(http.StatusOK, tplSwagger)
+	ctx.HTML(http.StatusOK, "swagger/openapi-viewer")
 }