Normalize dataset upload filenames so the UI can manage them

The dataset upload took Path(filename).name, which on POSIX does not split on a
backslash, so a Windows client sending a backslash path in the multipart filename
stored the name verbatim. The caption/thumbnail/delete endpoints then run it
through _safe_dataset_image_path, which rejects backslashes and '..', so the
labeling grid could list an image it could never preview, caption, or delete (an
orphan). Fold backslashes to forward slashes before taking the basename so the
true name is stored, and reject a basename that still contains '..' at upload
rather than persisting an unmanageable entry. Regression test covers a Windows
backslash path (stored and served under the clean basename) and a '..' rejection.

studio/backend/tests/test_diffusion_training.py

@@ -758,6 +758,34 @@ def test_diffusion_dataset_upload_accumulates(client, dataset_roots):
     assert r.json()["image_count"] == 3
 
 
+def test_diffusion_dataset_upload_normalizes_windows_and_rejects_dotdot(client, dataset_roots):
+    ds_root, _ = dataset_roots
+    # A Windows client can send a backslash path in the multipart filename; POSIX Path.name does
+    # not split on backslash, so it must be folded to the true basename, or the stored name holds
+    # backslashes that _safe_dataset_image_path later rejects -- an image the labeling grid can
+    # list but never preview/caption/delete (an orphan).
+    r = client.post(
+        "/api/train/diffusion/dataset",
+        data = {"name": "winset"},
+        files = [("files", ("C:\\Users\\me\\pics\\cat.png", b"png-bytes", "image/png"))],
+    )
+    assert r.status_code == 200, r.text
+    assert (ds_root / "winset" / "cat.png").read_bytes() == b"png-bytes"
+    # It is listed under the clean basename and the per-image endpoints accept it (not an orphan).
+    recs = client.get("/api/train/diffusion/dataset/winset/images").json()["images"]
+    assert any(rec["filename"] == "cat.png" for rec in recs)
+    assert client.get("/api/train/diffusion/dataset/winset/image/cat.png").status_code == 200
+
+    # A basename that still contains ".." (which _safe_dataset_image_path rejects) is refused at
+    # upload rather than persisted as an unmanageable entry.
+    r = client.post(
+        "/api/train/diffusion/dataset",
+        data = {"name": "winset"},
+        files = [("files", ("a..b.png", b"x", "image/png"))],
+    )
+    assert r.status_code == 400 and "Unsupported file" in r.json()["detail"]
+
+
 def test_diffusion_dataset_upload_over_cap_keeps_existing_example(
     client, dataset_roots, monkeypatch
 ):