studio: fix downloaded check -- compare basename not full path

The variant filename includes a subfolder prefix (e.g.
UD-Q4_K_XL/Kimi-K2.5-UD-Q4_K_XL-00001-of-00013.gguf) but rglob
returns just the filename. Use Path.name for the comparison.

studio/backend/routes/models.py

@@ -583,7 +583,7 @@ async def get_gguf_variants(
                     filename = v.filename,
                     quant = v.quant,
                     size_bytes = v.size_bytes,
-                    downloaded = v.filename in cached_files,
+                    downloaded = Path(v.filename).name in cached_files,
                 )
                 for v in variants
             ],