chore: wave-0 baseline (Tauri2+React shell, platform detect, fake catalog)

apps/desktop/src/pages/Backup.tsx

@@ -0,0 +1,19 @@
+import { Archive } from "lucide-react";
+
+/** 备份与迁移（PRD §7）：创建备份、查看内容、恢复。Wave 0 占位。 */
+export function BackupPage() {
+  return (
+    <div className="placeholder-page">
+      <div className="placeholder-icon">
+        <Archive size={40} strokeWidth={1.5} aria-hidden="true" />
+      </div>
+      <h2 className="placeholder-title">备份与迁移</h2>
+      <p className="placeholder-desc">
+        备份功能在 Wave 4 落地。备份将明确标注是否包含敏感信息，密钥永不进备份。
+      </p>
+      <button type="button" className="btn btn-secondary" disabled>
+        创建备份
+      </button>
+    </div>
+  );
+}