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>

modules/markup/render_test.go

@@ -0,0 +1,31 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package markup
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRenderIFrame(t *testing.T) {
+	render := func(ctx *RenderContext, opts ExternalRendererOptions) string {
+		sb := &strings.Builder{}
+		require.NoError(t, RenderIFrame(ctx, &opts, sb))
+		return sb.String()
+	}
+
+	ctx := NewRenderContext(t.Context()).
+		WithRelativePath("tree-path").
+		WithMetas(map[string]string{"user": "test-owner", "repo": "test-repo", "RefTypeNameSubURL": "src/branch/master"})
+
+	// the value is read from config RENDER_CONTENT_SANDBOX, empty means "disabled"
+	ret := render(ctx, ExternalRendererOptions{ContentSandbox: ""})
+	assert.Equal(t, `<iframe data-src="/test-owner/test-repo/render/src/branch/master/tree-path" class="external-render-iframe"></iframe>`, ret)
+
+	ret = render(ctx, ExternalRendererOptions{ContentSandbox: "allow"})
+	assert.Equal(t, `<iframe data-src="/test-owner/test-repo/render/src/branch/master/tree-path" class="external-render-iframe" sandbox="allow"></iframe>`, ret)
+}