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.

src/types/stats-widgets.ts

@@ -1,53 +1,9 @@
-export type WidgetType =
-  | "cpu" // CPU 使用率
-  | "memory" // 内存使用率
-  | "disk"; // 磁盘使用率
-// 预留未来功能
-// | 'network'   // 网络统计
-// | 'processes' // 进程数
-// | 'uptime';   // 运行时间
-
-export type WidgetSize = "small" | "medium" | "large";
-
-export interface Widget {
-  id: string; // 唯一 ID："cpu-1", "memory-2"
-  type: WidgetType; // 卡片类型
-  size: WidgetSize; // 尺寸：small/medium/large
-  x: number; // 网格X坐标 (0-11)
-  y: number; // 网格Y坐标
-  w: number; // 宽度（网格单位 1-12）
-  h: number; // 高度（网格单位）
-}
+export type WidgetType = "cpu" | "memory" | "disk";
 
 export interface StatsConfig {
-  widgets: Widget[];
+  enabledWidgets: WidgetType[];
 }
 
 export const DEFAULT_STATS_CONFIG: StatsConfig = {
-  widgets: [
-    { id: "cpu-1", type: "cpu", size: "medium", x: 0, y: 0, w: 4, h: 2 },
-    { id: "memory-1", type: "memory", size: "medium", x: 4, y: 0, w: 4, h: 2 },
-    { id: "disk-1", type: "disk", size: "medium", x: 8, y: 0, w: 4, h: 2 },
-  ],
+  enabledWidgets: ["cpu", "memory", "disk"],
 };
-
-export const WIDGET_TYPE_CONFIG = {
-  cpu: {
-    label: "CPU Usage",
-    defaultSize: { w: 4, h: 2 },
-    minSize: { w: 3, h: 2 },
-    maxSize: { w: 12, h: 4 },
-  },
-  memory: {
-    label: "Memory Usage",
-    defaultSize: { w: 4, h: 2 },
-    minSize: { w: 3, h: 2 },
-    maxSize: { w: 12, h: 4 },
-  },
-  disk: {
-    label: "Disk Usage",
-    defaultSize: { w: 4, h: 2 },
-    minSize: { w: 3, h: 2 },
-    maxSize: { w: 12, h: 4 },
-  },
-} as const;