refactor: simplify server stats widget system

Replaced complex drag-and-drop grid layout with simple checkbox-based
configuration and static responsive grid display.

- Removed react-grid-layout dependency and 6 related packages
- Simplified StatsConfig from complex Widget objects to simple array
- Added Statistics tab in HostManagerEditor for checkbox selection
- Refactored Server.tsx to use CSS Grid instead of ResponsiveGridLayout
- Simplified widget components by removing edit mode and size selection
- Deleted unused AddWidgetDialog and registry files
- Fixed statsConfig serialization in backend routes

Net result: -787 lines of code, cleaner architecture.

package.json

@@ -56,7 +56,6 @@
     "@types/jszip": "^3.4.0",
     "@types/multer": "^2.0.0",
     "@types/qrcode": "^1.5.5",
-    "@types/react-grid-layout": "^1.3.5",
     "@types/speakeasy": "^2.0.10",
     "@uiw/codemirror-extensions-langs": "^4.24.1",
     "@uiw/react-codemirror": "^4.24.1",
@@ -90,7 +89,6 @@
     "qrcode": "^1.5.4",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
-    "react-grid-layout": "^1.5.2",
     "react-h5-audio-player": "^3.10.1",
     "react-hook-form": "^7.60.0",
     "react-i18next": "^15.7.3",