feat(private_clipboard): 新增私有剪贴板文件访问接口并调整url返回逻辑

不再直接暴露本地文件路径作为文件url，改用api接口代理访问；新增对应路由处理文件的预览和下载请求，完善响应头配置。

src/server/private_clipboard.js

@@ -101,8 +101,7 @@ const saveFile = (raw, name, mime) => {
 }
 
 const formatRow = (row) => {
-  const filePath = row.file_path ? String(row.file_path) : ''
+  const url = row && row.id && row.file_path ? `/api/private_clipboard/file/${row.id}` : ''
-  const url = filePath ? '/' + filePath.replace(/^\/+/, '') : ''
   return {
     id: row.id,
     type: row.type,
@@ -402,6 +401,30 @@ const createRouter = () => {
   // ===== 以下业务路由需要登录 =====
   router.use(mustAuth)
 
+  router.get('/file/:id', (req, res) => {
+    try {
+      const id = Number.parseInt(String(req.params.id || ''), 10)
+      if (!id) return res.status(400).json({ ok: false, error: 'invalid_id' })
+      const row = getDb().prepare('SELECT id, type, file_name, file_mime, file_path FROM clipboard_items WHERE id = ?').get(id)
+      if (!row || !row.file_path) return res.status(404).json({ ok: false, error: 'file_not_found' })
+      const rel = String(row.file_path || '').replace(/^\/+/, '')
+      const abs = path.join(process.cwd(), rel)
+      if (!fs.existsSync(abs)) return res.status(404).json({ ok: false, error: 'file_not_found' })
+      const mime = String(row.file_mime || '').trim() || 'application/octet-stream'
+      res.setHeader('Content-Type', mime)
+      res.setHeader('Cache-Control', 'private, no-store, max-age=0')
+      const safeName = normalizeName(row.file_name || 'file')
+      const isImage = String(row.type || '').toLowerCase() === 'image' || mime.toLowerCase().startsWith('image/')
+      const dispositionType = isImage ? 'inline' : 'attachment'
+      const fallbackName = safeName.replace(/[^\x20-\x7E]/g, '_')
+      const utf8Name = encodeURIComponent(safeName)
+      res.setHeader('Content-Disposition', `${dispositionType}; filename="${fallbackName}"; filename*=UTF-8''${utf8Name}`)
+      return res.sendFile(abs)
+    } catch (e) {
+      return res.status(500).json({ ok: false, error: String(e.message || e) })
+    }
+  })
+
   router.get('/list', (req, res) => {
     try {
       const pageSizeRaw = String((req.query && req.query.page_size) || '50').trim().toLowerCase()