Fix hardcoded text and add missing i18n translations in file manager

- Add 18 new translation keys for file manager sidebar and context menu operations
- Replace hardcoded Chinese text with t() function calls in FileManagerSidebar.tsx:
  * Toast messages for remove/unpin/clear operations
  * Context menu items for recent files, pinned files, and shortcuts
- Replace hardcoded Chinese text with t() function calls in FileManagerContextMenu.tsx:
  * Pin/unpin file menu items
  * Add to shortcuts menu item
  * Save to system menu items with dynamic count support
- Add bilingual support for all new strings (English and Chinese)
- Improve consistency with existing i18n patterns

src/locales/zh/translation.json

@@ -736,7 +736,25 @@
     "cannotSaveFile": "无法保存文件",
     "starred": "收藏",
     "shortcuts": "快捷方式",
-    "directories": "目录"
+    "directories": "目录",
+    "removedFromRecentFiles": "已从最近访问中移除\"{{name}}\"",
+    "removeFailed": "移除失败",
+    "unpinnedSuccessfully": "已取消固定\"{{name}}\"",
+    "unpinFailed": "取消固定失败",
+    "removedShortcut": "已移除快捷方式\"{{name}}\"",
+    "removeShortcutFailed": "移除快捷方式失败",
+    "clearedAllRecentFiles": "已清除所有最近访问记录",
+    "clearFailed": "清除失败",
+    "removeFromRecentFiles": "从最近访问中移除",
+    "clearAllRecentFiles": "清除所有最近访问",
+    "unpinFile": "取消固定",
+    "removeShortcut": "移除快捷方式",
+    "saveFilesToSystem": "保存 {{count}} 个文件到系统",
+    "saveToSystem": "保存到系统",
+    "pinFile": "固定文件",
+    "addToShortcuts": "添加到快捷方式",
+    "selectLocationToSave": "选择位置保存",
+    "downloadToDefaultLocation": "下载到默认位置"
   },
   "tunnels": {
     "title": "SSH 隧道",