refactor: 提取 Button 组件，统一按钮变体、尺寸和加载状态

新增 Button.tsx 支持 5 种变体(primary/secondary/danger/ghost/purple)、
3 种尺寸(sm/md/lg)、pill/rounded 形状及内置 loading 状态，
替换 8 个文件中 16 处重复的按钮样板代码。

src/components/QrInviteModal.tsx

@@ -6,6 +6,7 @@ import { X, Copy, Share2, QrCode } from "lucide-react";
 import type { SceneType } from "@/types";
 import { getSceneConfig } from "@/lib/sceneConfig";
 import Modal from "@/components/Modal";
+import Button from "@/components/Button";
 import { useToast } from "@/hooks/useToast";
 
 interface QrInviteModalProps {
@@ -92,20 +93,23 @@ export default function QrInviteModal({
         </div>
 
         <div className="mt-5 flex w-full gap-2.5">
-          <button
+          <Button
             onClick={handleCopy}
-            className="flex h-11 flex-1 items-center justify-center gap-1.5 rounded-xl bg-elevated text-sm font-semibold text-secondary ring-1 ring-border transition-colors active:bg-subtle"
+            variant="secondary"
+            size="lg"
+            icon={<Copy size={15} />}
+            className="flex-1"
           >
-            <Copy size={15} />
             复制链接
-          </button>
-          <button
+          </Button>
+          <Button
             onClick={handleShare}
-            className="flex h-11 flex-1 items-center justify-center gap-1.5 rounded-xl bg-accent text-sm font-semibold text-white shadow-lg shadow-accent/20 transition-colors active:bg-accent-hover"
+            size="lg"
+            icon={<Share2 size={15} />}
+            className="flex-1"
           >
-            <Share2 size={15} />
             发送邀请
-          </button>
+          </Button>
         </div>
       </div>
     </Modal>