fix: 预览时本地图片不显示

问题: 重构后丢失了原始版本的图片路径转换逻辑，
      相对路径图片（如 ![](./images/logo.png)）无法显示。

修复:
- 新增 rehypeFixImages 自定义 rehype 插件
- 在 sanitize 之后、highlight 之前执行
- 将相对路径 src 转换为 file:// 绝对路径
- 基于当前文件路径计算目录，拼接图片路径
- renderMarkdown 新增 filePath 可选参数
- Preview 组件传入 activeTab.filePath

示例:
  Markdown: ![](./images/logo.png)
  文件路径: /project/docs/README.md
  转换后:   file:///project/docs/images/logo.png

src/renderer/components/Preview/Preview.tsx

@@ -19,7 +19,7 @@ export function Preview() {
     }
 
     const requestId = ++requestIdRef.current
-    renderMarkdown(activeTab.content).then(result => {
+    renderMarkdown(activeTab.content, activeTab.filePath).then(result => {
       // M-07: 只有当 requestId 仍为最新时才更新
       if (requestId === requestIdRef.current) {
         setHtml(result)

src/renderer/lib/markdown.ts

@@ -1,4 +1,4 @@
-import { unified } from 'unified'
+import { unified, type Plugin } from 'unified'
 import remarkParse from 'remark-parse'
 import remarkGfm from 'remark-gfm'
 import remarkRehype from 'remark-rehype'
@@ -6,7 +6,42 @@ import rehypeRaw from 'rehype-raw'
 import rehypeSanitize, { defaultSchema } from 'rehype-sanitize'
 import rehypeStringify from 'rehype-stringify'
 import rehypeHighlight from 'rehype-highlight'
+import type { Element, Root } from 'hast'
 
+// 自定义 rehype 插件：将相对路径图片转为 file:// 绝对路径
+function rehypeFixImages(filePath: string | null): Plugin<[], Root> {
+  return () => (tree: Root) => {
+    if (!filePath) return
+
+    // 获取文件所在目录
+    const dir = filePath.replace(/[/\\][^/\\]+$/, '')
+    const sep = dir.includes('\\') ? '\\' : '/'
+
+    function visit(node: Element | Root): void {
+      if (!node.children) return
+      for (const child of node.children) {
+        if (child.type === 'element' && child.tagName === 'img') {
+          const src = child.properties?.src as string | undefined
+          if (src && !src.startsWith('http://') && !src.startsWith('https://') && !src.startsWith('data:') && !src.startsWith('file://')) {
+            // 相对路径转绝对路径
+            child.properties = {
+              ...child.properties,
+              src: 'file://' + (dir + sep + src).replace(/\\/g, '/')
+            }
+          }
+        }
+        if (child.type === 'element') {
+          visit(child as Element)
+        }
+      }
+    }
+
+    visit(tree)
+  }
+}
+
+export async function renderMarkdown(content: string, filePath?: string | null): Promise<string> {
+  try {
     const processor = unified()
       .use(remarkParse)
       .use(remarkGfm)
@@ -19,11 +54,10 @@ const processor = unified()
           img: [...(defaultSchema.attributes?.img ?? []), ['src']]
         }
       })
+      .use(rehypeFixImages(filePath ?? null))
       .use(rehypeHighlight)
       .use(rehypeStringify)
 
-export async function renderMarkdown(content: string): Promise<string> {
-  try {
     const result = await processor.process(content)
     return String(result)
   } catch (e) {