fix(gallery): address review findings in zip download module

- Handle downloadImagesAsZip return value and surface errors to user
- Fix inferImageExtension garbage output for data URIs without semicolons
- Scale blob URL revocation delay for large zip files (>5MB → 60s)
- Cap uniqueZipPath dedup loop at 10k iterations with timestamp fallback
- Support relative URLs in inferImageExtension via base URL
- Handle svg+xml MIME subtype correctly

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/gallery/page.tsx

@@ -858,7 +858,14 @@ function GalleryInner() {
           name: `infiplot-branch-${String(branchN).padStart(3, "0")}.${inferImageExtension(alt.imageUrl)}`,
         });
       }
-      await downloadImagesAsZip(files, `infiplot-gallery-${doc.id}.zip`);
+      const result = await downloadImagesAsZip(files, `infiplot-gallery-${doc.id}.zip`);
+      if (result.downloaded === 0) {
+        alert("所有图片抓取失败，请检查网络后重试");
+      } else if (result.failed.length > 0) {
+        alert(`已打包 ${result.downloaded} 张，${result.failed.length} 张抓取失败`);
+      }
+    } catch {
+      alert("打包下载失败，请重试");
     } finally {
       setDownloadingScenes(false);
     }