refactor: 用 marked.js 替换手写 Markdown 渲染器

- 引入 marked.js（~40KB）作为专业 Markdown 解析引擎
- markdown.js 精简为 20 行薄封装，保留 renderMarkdown 兼容别名
- CSS 从 .md-* 选择器改为标准 HTML 标签选择器
- 支持 GFM（表格、任务列表、删除线）、代码块语法高亮
- 天然处理流式不完整 Markdown（未闭合代码块等）

static/css/style.css

@@ -49,7 +49,7 @@
 }
 
 /* Markdown rendered content (inside review suggestions) */
-.review-content .md-code-block {
+.review-content pre {
     background: #1e293b;
     color: #e2e8f0;
     padding: 12px 14px;
@@ -60,16 +60,13 @@
     line-height: 1.5;
     font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
 }
-.review-content .md-code-block[data-lang]::before {
-    content: attr(data-lang);
-    display: block;
-    font-size: 10px;
-    color: #94a3b8;
-    margin-bottom: 4px;
-    text-transform: uppercase;
-    letter-spacing: 0.05em;
+.review-content pre code {
+    background: none;
+    color: inherit;
+    padding: 0;
+    font-size: inherit;
 }
-.review-content .md-inline-code {
+.review-content code {
     background: #f1f5f9;
     padding: 1px 5px;
     border-radius: 4px;
@@ -77,40 +74,40 @@
     font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
     color: #be185d;
 }
-.review-content .md-p {
+.review-content p {
     margin: 6px 0;
     line-height: 1.6;
 }
-.review-content .md-h1, .review-content .md-h2, .review-content .md-h3 {
+.review-content h1, .review-content h2, .review-content h3 {
     font-weight: 600;
     margin: 10px 0 4px;
 }
-.review-content .md-h1 { font-size: 1.2em; }
-.review-content .md-h2 { font-size: 1.1em; }
-.review-content .md-h3 { font-size: 1em; }
-.review-content .md-ul, .review-content .md-ol {
+.review-content h1 { font-size: 1.2em; }
+.review-content h2 { font-size: 1.1em; }
+.review-content h3 { font-size: 1em; }
+.review-content ul, .review-content ol {
     margin: 6px 0;
     padding-left: 20px;
 }
-.review-content .md-li {
+.review-content li {
     margin: 2px 0;
     line-height: 1.5;
 }
-.review-content .md-link {
+.review-content a {
     color: #2563eb;
     text-decoration: underline;
 }
-.review-content .md-link:hover {
+.review-content a:hover {
     color: #1d4ed8;
 }
-.review-content .md-blockquote {
+.review-content blockquote {
     border-left: 3px solid #d1d5db;
     padding-left: 12px;
     color: #6b7280;
     margin: 8px 0;
     font-style: italic;
 }
-.review-content .md-hr {
+.review-content hr {
     border: none;
     border-top: 1px solid #e5e7eb;
     margin: 12px 0;