feat: Markdown 工具栏编辑器与服务端预览

共用 md_editor 于发帖/改帖/评论，扩展 ComposeBodyToHTML，并提供 /compose/preview。

Co-authored-by: Cursor <cursoragent@cursor.com>

services/compose_body_test.go

@@ -0,0 +1,72 @@
+package services
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestComposeBodyToHTML_MarkdownBasics(t *testing.T) {
+	in := strings.TrimSpace(`
+## 标题
+
+段落 **粗体** 与 *斜体* 和 ~~删除~~ 与 ` + "`code`" + `。
+
+- 甲
+- 乙
+
+1. 一
+2. 二
+
+> 引用行
+
+` + "```go\nfmt.Println(1)\n```" + `
+
+[链接](https://example.com)
+![图](/uploads/a.png)
+`)
+	out := ComposeBodyToHTML(in)
+	checks := []string{
+		"<h2>", "标题", "</h2>",
+		"<strong>粗体</strong>",
+		"<em>斜体</em>",
+		"<s>删除</s>",
+		"<code>code</code>",
+		"<ul>", "<li>",
+		"<ol>",
+		"<blockquote>",
+		`class="language-go"`,
+		"fmt.Println(1)",
+		`href="https://example.com"`,
+		`src="/uploads/a.png"`,
+	}
+	for _, c := range checks {
+		if !strings.Contains(out, c) {
+			t.Fatalf("missing %q in:\n%s", c, out)
+		}
+	}
+	if strings.Contains(out, "<h1>") {
+		t.Fatal("must not emit h1")
+	}
+}
+
+func TestComposeBodyToHTML_PassThroughGates(t *testing.T) {
+	in := `<members-only data-gate="login"><p>隐</p></members-only>`
+	if got := ComposeBodyToHTML(in); got != in {
+		t.Fatalf("gate html should pass through, got %q", got)
+	}
+}
+
+func TestComposeBodyToHTML_H1MapsToH2(t *testing.T) {
+	out := ComposeBodyToHTML("# 顶标题")
+	if !strings.Contains(out, "<h2>") || strings.Contains(out, "<h1>") {
+		t.Fatalf("got %s", out)
+	}
+}
+
+func TestHTMLToComposePlain_RoundTripLite(t *testing.T) {
+	html := ComposeBodyToHTML("## Hi\n\n**x**")
+	plain := HTMLToComposePlain(html)
+	if !strings.Contains(plain, "## Hi") || !strings.Contains(plain, "**x**") {
+		t.Fatalf("plain=%q", plain)
+	}
+}