fix(sidebar): let a ghost folder be cleared from the UI (#1145)

Deleting a folder a file manager had already moved away answered "Folder no longer
exists" and stopped there: `remove_known_folder()` only ran after `shutil.rmtree()`
succeeded, so the records keeping the node in the tree could never be dropped, and
the modal's missing state hid its confirm button. The sidebar was a dead end for
exactly the case the folder list cannot self-heal.

* `ModelMoveService.forget_folder()` drops a folder's records (the recorded entry
  plus the cached models under it) and refuses with `code: "exists"` while any root
  still holds the directory, because then the caller wants a delete.
* `delete_folder()` forgets the records instead of failing when the directory is
  already gone. The dry run stays a pure read and keeps answering `missing`, which
  is what makes the dialog offer the removal. `POST`/`GET
  /api/lm/{prefix}/forget-folder` is the endpoint for a node that maps to no
  directory at all (the GET form is for the browser extension, which is GET-only).
* The sidebar falls back to it from the delete flow, offers "Remove from list" in
  the missing state, and reports the folder as removed from the sidebar rather than
  as a delete that never happened. An `exists` conflict says the directory is back
  and should be deleted instead.

Verified: 3811 backend tests, 1543 JS tests, 96 Vue tests, 20 i18n tests.

tests/services/test_model_file_service.py

@@ -400,14 +400,74 @@ async def test_delete_folder_rejects_paths_outside_roots(tmp_path: Path):
 
 
 @pytest.mark.asyncio
-async def test_delete_folder_reports_missing_directory(tmp_path: Path):
+async def test_delete_folder_forgets_a_directory_that_is_already_gone(tmp_path: Path):
+    """There is nothing to delete, but the sidebar node can still be cleared.
+
+    A folder moved with a file manager leaves its records behind; answering
+    "Folder no longer exists" left the ghost with no way out of the UI (#1145).
+    """
+    scanner = FakeScanner([tmp_path])
+    service = ModelMoveService(scanner, "lora")
+    gone = tmp_path / "gone"
+
+    result = await service.delete_folder(str(gone))
+
+    assert result["success"] is True
+    assert result["code"] == "missing"
+    assert result["already_missing"] is True
+    assert result["folder"] == "gone"
+    assert scanner.removed_folders == ["gone"]
+    assert scanner.removed_folder_paths == [str(gone)]
+
+
+@pytest.mark.asyncio
+async def test_delete_folder_dry_run_still_reports_a_missing_directory(tmp_path: Path):
+    """The probe stays a pure read — it is what makes the dialog offer the removal."""
     scanner = FakeScanner([tmp_path])
     service = ModelMoveService(scanner, "lora")
 
-    result = await service.delete_folder(str(tmp_path / "gone"))
+    result = await service.delete_folder(str(tmp_path / "gone"), dry_run=True)
 
     assert result["success"] is False
-    assert "no longer exists" in result["error"]
+    assert result["code"] == "missing"
+    assert scanner.removed_folders == []
+
+
+@pytest.mark.asyncio
+async def test_forget_folder_drops_the_records_of_a_gone_directory(tmp_path: Path):
+    scanner = FakeScanner([tmp_path])
+    service = ModelMoveService(scanner, "lora")
+
+    result = await service.forget_folder("Krea2/Women")
+
+    assert result["success"] is True
+    assert result["folder"] == "Krea2/Women"
+    # No absolute path: the same relative folder may exist under several roots,
+    # and the scanner keeps an entry another root still holds.
+    assert scanner.removed_folders == ["Krea2/Women"]
+    assert scanner.removed_folder_paths == [None]
+
+
+@pytest.mark.asyncio
+async def test_forget_folder_refuses_while_the_directory_still_exists(tmp_path: Path):
+    """Then the caller wants a delete, not a cleanup."""
+    (tmp_path / "Krea2" / "Women").mkdir(parents=True)
+    scanner = FakeScanner([tmp_path])
+    service = ModelMoveService(scanner, "lora")
+
+    result = await service.forget_folder("Krea2/Women")
+
+    assert result["success"] is False
+    assert result["code"] == "exists"
+    assert scanner.removed_folders == []
+
+
+@pytest.mark.asyncio
+async def test_forget_folder_requires_a_folder(tmp_path: Path):
+    service = ModelMoveService(FakeScanner([tmp_path]), "lora")
+
+    assert (await service.forget_folder(""))["success"] is False
+    assert (await service.forget_folder("../outside"))["success"] is False
 
 
 @pytest.mark.asyncio