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/routes/test_model_source_handlers.py

@@ -403,9 +403,7 @@ def test_find_matching_root_includes_other_roots(isolated_config_roots, tmp_path
     vae_root = tmp_path / "vae"
     isolated_config_roots.other_roots = [str(vae_root)]
 
-    assert model_source_handlers._find_matching_root(str(vae_root / "sub")) == str(
-        vae_root
-    )
+    assert model_source_handlers._find_matching_root(str(vae_root / "sub")) == str(vae_root).replace(os.sep, "/").replace(os.sep, "/")
 
 
 def test_find_matching_root_respects_path_boundaries(isolated_config_roots, tmp_path):