refactor: 拆分 ShareCardModal（951 行 → 4 个模块）

- ShareCardModal.tsx (199 行): 模态框编排逻辑（图片生成、保存、分享）
- RestaurantShareCard.tsx: 餐厅分享卡片纯展示组件
- BlindboxShareCard.tsx: 盲盒分享卡片纯展示组件
- shareImage.ts: 图片工具函数（toPng 封装、dataUrl 转换、下载）

src/lib/shareImage.ts

@@ -0,0 +1,52 @@
+import { toPng } from "html-to-image";
+
+export async function loadImageAsDataUrl(src: string): Promise<string | null> {
+  try {
+    const img = new Image();
+    img.crossOrigin = "anonymous";
+    img.src = src;
+    await new Promise<void>((resolve, reject) => {
+      img.onload = () => resolve();
+      img.onerror = () => reject();
+    });
+    const canvas = document.createElement("canvas");
+    canvas.width = img.naturalWidth;
+    canvas.height = img.naturalHeight;
+    const ctx = canvas.getContext("2d")!;
+    ctx.drawImage(img, 0, 0);
+    return canvas.toDataURL("image/jpeg", 0.85);
+  } catch {
+    return null;
+  }
+}
+
+export async function generateImage(el: HTMLElement): Promise<string> {
+  return toPng(el, {
+    pixelRatio: 2,
+    quality: 0.95,
+    cacheBust: true,
+    skipAutoScale: true,
+    filter: (node) => {
+      if (node instanceof HTMLElement && node.dataset.shareExclude === "true") {
+        return false;
+      }
+      return true;
+    },
+  });
+}
+
+export function downloadDataUrl(dataUrl: string, filename: string) {
+  const link = document.createElement("a");
+  link.download = filename;
+  link.href = dataUrl;
+  link.click();
+}
+
+export function dataUrlToFile(dataUrl: string, filename: string): File {
+  const arr = dataUrl.split(",");
+  const mime = arr[0].match(/:(.*?);/)?.[1] || "image/png";
+  const bstr = atob(arr[1]);
+  const u8 = new Uint8Array(bstr.length);
+  for (let i = 0; i < bstr.length; i++) u8[i] = bstr.charCodeAt(i);
+  return new File([u8], filename, { type: mime });
+}