test: make the backend suite pass on Windows

The new informational Windows job surfaced 91 failures: four real bugs (fixed in
the previous two commits) and a long tail of POSIX assumptions in the tests
themselves. None of them assert behaviour that differs on Windows, so they are
fixed rather than skipped wherever the behaviour is the same:

* Path spelling: expectations built with `str(tmp_path)` are compared through an
  identity helper (`normalize_root_identity`) or the platform's separator, since
  the code hands out forward-slashed business paths while `str()` is native.
* Root labels: `_root_display_labels` prefixes the drive on Windows ("C: loras"),
  so label assertions compare the final path component or derive the expected
  label from the same helper.
* `~` expansion reads USERPROFILE (ntpath), not HOME; timer granularity rounds
  0.1s down; a case-only rename counts as "already named" via `os.path.normcase`.
* Genuinely POSIX-only strategies are skipped with a reason instead of failing:
  the headless clipboard fallback, dbus file-manager selection, xdg-open, and
  the case-fold index test that simulates a case-sensitive filesystem.

tests/services/test_model_scanner.py

@@ -48,6 +48,20 @@ class RecordingWebSocketManager:
         self.broadcasts.append(payload)
 
 
+def _root_label_names(labels) -> List[str]:
+    """The root names behind display labels.
+
+    ``_root_display_labels`` prefixes the drive on Windows ("C: loras") and may
+    grow a label leftwards to keep it unique ("parent/loras"), so a test that only
+    cares *which* root was walked compares the final path component.
+    """
+
+    return [
+        label.replace("\\", "/").rstrip("/").rsplit("/", 1)[-1].rsplit(" ", 1)[-1]
+        for label in labels
+    ]
+
+
 def _normalize_path(path: Path) -> str:
     return str(path).replace(os.sep, "/")
 
@@ -2306,6 +2320,10 @@ def test_count_cached_entries_per_root_uses_longest_prefix():
 
 
 @pytest.mark.asyncio
+@pytest.mark.skipif(
+    os.name == "nt",
+    reason="simulates a case-sensitive filesystem; Windows matches case itself",
+)
 async def test_reconcile_case_fold_fallback_is_indexed_not_linear(
     tmp_path: Path, monkeypatch
 ):
@@ -2465,7 +2483,10 @@ async def test_reconcile_walk_broadcasts_per_root_progress(tmp_path: Path, monke
     assert len(walk_messages) >= 2
 
     # Intermediate ticks name the root being walked.
-    assert any(message["active_roots"] == ["loras"] for message in walk_messages)
+    assert any(
+        _root_label_names(message["active_roots"]) == ["loras"]
+        for message in walk_messages
+    )
 
     final = walk_messages[-1]
     assert final["files_seen"] == 4
@@ -2546,7 +2567,7 @@ async def test_reconcile_scoped_scan_leaves_other_roots_untouched(tmp_path: Path
     assert summary is not None
     assert summary["added"] == 1
     assert summary["removed"] == 0
-    assert summary["scanned_roots"] == ["a"]
+    assert _root_label_names(summary["scanned_roots"]) == ["a"]
 
     cached_paths = {item["file_path"] for item in scanner._cache.raw_data}
     assert cached_paths == {
@@ -2717,11 +2738,13 @@ async def test_reconcile_reports_root_filtered_out_at_startup(tmp_path: Path, mo
     assert summary["added"] == 0
     assert summary["removed"] == 0
     # ...but the switched-off root is no longer silent: it is reported with the
-    # entries that were kept for it.
+    # entries that were kept for it. Its label is what the shared label helper
+    # makes of the pair (drive-prefixed on Windows).
+    labels = model_scanner._root_display_labels([str(_primary), str(missing)])
     assert summary["skipped_roots"] == [
         {
             "path": _normalize_path(missing),
-            "label": missing.name,
+            "label": labels[str(missing)],
             "reason": "root_unavailable",
         }
     ]
@@ -2746,7 +2769,7 @@ async def test_scoped_scan_stays_quiet_about_filtered_out_roots(tmp_path: Path,
     assert summary is not None
     # The caller asked for one root: a root it did not ask about is not its
     # business, but its entries are still kept.
-    assert summary["scanned_roots"] == [primary.name]
+    assert _root_label_names(summary["scanned_roots"]) == [primary.name]
     assert "root_unavailable" not in {
         entry["reason"] for entry in summary["skipped_roots"]
     }
@@ -3091,7 +3114,7 @@ async def test_scoped_scan_accepts_the_host_path_spelling(tmp_path: Path, monkey
     summary = await scanner._reconcile_cache(scope=scope)
 
     assert summary is not None
-    assert summary["scanned_roots"] == [primary.name]
+    assert _root_label_names(summary["scanned_roots"]) == [primary.name]
 
 
 @pytest.mark.asyncio