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/utils/test_metadata_manager.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 import json
 
 import pytest
+import os
 
 from py.utils.metadata_manager import MetadataManager
 from py.utils.models import LoraMetadata
@@ -174,7 +175,7 @@ async def test_load_metadata_payload_tolerates_missing_model_file(tmp_path) -> N
     """A nonexistent model file must not crash payload loading."""
     missing_path = tmp_path / "Ghost.safetensors"
     payload = await MetadataManager.load_metadata_payload(str(missing_path))
-    assert payload["file_path"] == str(missing_path)
+    assert payload["file_path"] == str(missing_path).replace(os.sep, "/")
     assert "file_name" not in payload
     assert "size" not in payload
     assert "modified" not in payload