feat(client): UI primitives 基线（E2）

- 引入 Radix Dialog/DropdownMenu/Tabs + CVA
- 新增 Button/Input/Dialog/Tabs/Table/Card 等组件（本地拷贝单向同步）
- 增加 touch-target(44px) 样式基础

client/src/components/ui/dropdown-menu.tsx

@@ -0,0 +1,54 @@
+import * as React from "react";
+import * as DropdownMenuPrimitive from "@radix-ui/react-dropdown-menu";
+import { cn } from "../../lib/utils";
+
+export const DropdownMenu = DropdownMenuPrimitive.Root;
+export const DropdownMenuTrigger = DropdownMenuPrimitive.Trigger;
+export const DropdownMenuContent = React.forwardRef<
+  React.ElementRef<typeof DropdownMenuPrimitive.Content>,
+  React.ComponentPropsWithoutRef<typeof DropdownMenuPrimitive.Content>
+>(({ className, sideOffset = 8, ...props }, ref) => (
+  <DropdownMenuPrimitive.Portal>
+    <DropdownMenuPrimitive.Content
+      ref={ref}
+      sideOffset={sideOffset}
+      className={cn(
+        "z-50 min-w-[12rem] overflow-hidden rounded-xl border border-slate-200 bg-white p-1 text-slate-900 shadow-md",
+        className,
+      )}
+      {...props}
+    />
+  </DropdownMenuPrimitive.Portal>
+));
+
+DropdownMenuContent.displayName = "DropdownMenuContent";
+
+export const DropdownMenuItem = React.forwardRef<
+  React.ElementRef<typeof DropdownMenuPrimitive.Item>,
+  React.ComponentPropsWithoutRef<typeof DropdownMenuPrimitive.Item>
+>(({ className, ...props }, ref) => (
+  <DropdownMenuPrimitive.Item
+    ref={ref}
+    className={cn(
+      "flex cursor-default select-none items-center rounded-lg px-3 py-2 text-sm outline-none focus:bg-slate-100 data-[disabled]:pointer-events-none data-[disabled]:opacity-50",
+      className,
+    )}
+    {...props}
+  />
+));
+
+DropdownMenuItem.displayName = "DropdownMenuItem";
+
+export const DropdownMenuSeparator = React.forwardRef<
+  React.ElementRef<typeof DropdownMenuPrimitive.Separator>,
+  React.ComponentPropsWithoutRef<typeof DropdownMenuPrimitive.Separator>
+>(({ className, ...props }, ref) => (
+  <DropdownMenuPrimitive.Separator
+    ref={ref}
+    className={cn("-mx-1 my-1 h-px bg-slate-200", className)}
+    {...props}
+  />
+));
+
+DropdownMenuSeparator.displayName = "DropdownMenuSeparator";
+