fix: 统一所有弹框组件 padding + 主题风格

- ModalOverlay: 内容容器加 p-6 + border + shadow-xl
- alert-dialog: 统一主题变量 bg-[var(--dg-bg-secondary)] + rounded-xl
- sheet: SheetContent 加 p-6，移除 Header/Footer 重复 p-4
- drawer: DrawerContent 加 p-6，移除 Header/Footer 重复 p-4
- 所有弹框统一: p-6 padding + dg-border + shadow-xl + z-[101]

frontend/src/components/ui/alert-dialog.tsx

@@ -52,7 +52,7 @@ function AlertDialogContent({
       <AlertDialogPrimitive.Content
         data-slot="alert-dialog-content"
         className={cn(
-          "bg-background data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95 fixed top-[50%] left-[50%] z-50 grid w-full max-w-[calc(100%-2rem)] translate-x-[-50%] translate-y-[-50%] gap-4 rounded-lg border p-6 shadow-lg duration-200 sm:max-w-lg",
+          "bg-[var(--dg-bg-secondary)] text-[var(--dg-text-primary)] border border-[var(--dg-border)] data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-95 fixed top-[50%] left-[50%] z-[101] grid w-full max-w-[calc(100%-2rem)] -translate-x-1/2 -translate-y-1/2 gap-4 rounded-xl p-6 shadow-xl duration-200 sm:max-w-lg",
           className
         )}
         {...props}