feat(04-02): add downloadFile and downloadZip utility functions

- src/utils/downloadFile.ts: Blob + createObjectURL + anchor click download
- src/utils/downloadZip.ts: JSZip archive builder with same Blob download pattern
- npm install jszip ^3.10.1 (only new runtime dependency in phase 4)

src/utils/downloadFile.ts

@@ -0,0 +1,12 @@
+// src/utils/downloadFile.ts
+// Triggers a browser file download from a string.
+// Pattern: MDN URL.createObjectURL — https://developer.mozilla.org/en-US/docs/Web/API/URL/createObjectURL_static
+export function downloadFile(content: string, filename: string): void {
+  const blob = new Blob([content], { type: 'text/plain;charset=utf-8' });
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = filename;
+  a.click();
+  URL.revokeObjectURL(url);
+}