新增：Markdown 本地文件链接支持 + Shell 语法高亮

Markdown 预览增强：
- 支持点击本地文件链接（相对路径）打开对应文件
- 支持链接文本中的加粗/斜体等内联语法
- 锚点链接保持页面内跳转，外部链接新窗口打开

代码高亮增强：
- 添加 sh/bash/shell 语言别名映射
- 安装 @codemirror/legacy-modes 支持 .sh 文件语法高亮

web/package-lock.json

@@ -24,6 +24,7 @@
         "@codemirror/lang-sql": "^6.10.0",
         "@codemirror/lang-yaml": "^6.1.2",
         "@codemirror/language": "^6.12.1",
+        "@codemirror/legacy-modes": "^6.5.2",
         "@codemirror/state": "^6.5.3",
         "@codemirror/theme-one-dark": "^6.1.3",
         "@codemirror/view": "^6.39.8",
@@ -393,6 +394,15 @@
         "style-mod": "^4.0.0"
       }
     },
+    "node_modules/@codemirror/legacy-modes": {
+      "version": "6.5.2",
+      "resolved": "https://registry.npmmirror.com/@codemirror/legacy-modes/-/legacy-modes-6.5.2.tgz",
+      "integrity": "sha512-/jJbwSTazlQEDOQw2FJ8LEEKVS72pU0lx6oM54kGpL8t/NJ2Jda3CZ4pcltiKTdqYSRk3ug1B3pil1gsjA6+8Q==",
+      "license": "MIT",
+      "dependencies": {
+        "@codemirror/language": "^6.0.0"
+      }
+    },
     "node_modules/@codemirror/lint": {
       "version": "6.9.2",
       "resolved": "https://registry.npmmirror.com/@codemirror/lint/-/lint-6.9.2.tgz",