Return empty string instead of undefined from getFileName on not-found

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

tests/stores/compare.test.js

@@ -93,14 +93,14 @@ describe("compareStore", () => {
       expect(result).toBe("file1.csv");
     });
 
-    it("returns undefined for non-existent id", async () => {
+    it("returns empty string for non-existent id", async () => {
       mockGet.mockResolvedValue({
         data: [{ id: 1, name: "file1.csv" }],
       });
 
       const result = await store.getFileName(99);
 
-      expect(result).toBeUndefined();
+      expect(result).toBe("");
     });
   });
 });