feat: refactor server stats widgets into modular structure

Created dedicated widgets directory with individual components:
- CpuWidget, MemoryWidget, DiskWidget as separate components
- Widget registry for centralized widget configuration
- AddWidgetDialog for user-friendly widget selection
- Updated Server.tsx to use modular widget system

Benefits:
- Better code organization and maintainability
- Easier to add new widget types in the future
- Centralized widget metadata and configuration
- User can now add widgets via dialog interface

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/locales/zh/translation.json

@@ -1141,6 +1141,7 @@
     "available": "可用",
     "editLayout": "编辑布局",
     "cancelEdit": "取消",
+    "addWidget": "添加小组件",
     "saveLayout": "保存布局",
     "unsavedChanges": "有未保存的更改",
     "layoutSaved": "布局保存成功",