feat(sidebar): show empty folders and create folders from the sidebar (#999)

Empty folders (tracked in the scan-recorded all_folders list, same source
the move/download destination picker uses) can now be surfaced in the
folder sidebar via a view-options toggle, dimmed when their subtree holds
no models. Folders can be created directly from the sidebar through a new
POST /api/lm/{prefix}/create-folder endpoint with library-root
containment checks; the scanner records the new directory incrementally
so the tree reflects it without a rescan.

The sidebar header moves its view toggles (tree/list, recursive, empty
folders) into a "..." menu to fit the new create-folder button.

tests/routes/test_model_move_handler.py

@@ -0,0 +1,93 @@
+import json
+import logging
+
+import pytest
+
+from py.routes.handlers.model_handlers import ModelMoveHandler
+
+
+class FakeMoveService:
+    def __init__(self, result):
+        self._result = result
+        self.received_path = None
+
+    async def create_folder(self, folder_path):
+        self.received_path = folder_path
+        return self._result
+
+
+class FakeRequest:
+    def __init__(self, payload):
+        self._payload = payload
+
+    async def json(self):
+        return self._payload
+
+
+def _make_handler(result):
+    service = FakeMoveService(result)
+    handler = ModelMoveHandler(
+        move_service=service, logger=logging.getLogger(__name__)
+    )
+    return handler, service
+
+
+@pytest.mark.asyncio
+async def test_create_folder_success():
+    handler, service = _make_handler(
+        {"success": True, "folder": "characters/anime", "created": True}
+    )
+
+    response = await handler.create_folder(
+        FakeRequest({"folder_path": "/library/characters/anime"})
+    )
+
+    assert response.status == 200
+    payload = json.loads(response.text)
+    assert payload["success"] is True
+    assert payload["folder"] == "characters/anime"
+    assert service.received_path == "/library/characters/anime"
+
+
+@pytest.mark.asyncio
+async def test_create_folder_missing_path():
+    handler, service = _make_handler({"success": True})
+
+    response = await handler.create_folder(FakeRequest({}))
+
+    assert response.status == 400
+    payload = json.loads(response.text)
+    assert payload["success"] is False
+    assert service.received_path is None
+
+
+@pytest.mark.asyncio
+async def test_create_folder_service_failure_maps_to_400():
+    handler, _service = _make_handler(
+        {
+            "success": False,
+            "error": "Folder path '/etc/evil' is outside configured library directories",
+        }
+    )
+
+    response = await handler.create_folder(FakeRequest({"folder_path": "/etc/evil"}))
+
+    assert response.status == 400
+    payload = json.loads(response.text)
+    assert payload["success"] is False
+    assert "outside configured library" in payload["error"]
+
+
+@pytest.mark.asyncio
+async def test_create_folder_invalid_json_body():
+    class BadJsonRequest:
+        async def json(self):
+            raise ValueError("bad json")
+
+    handler, _service = _make_handler({"success": True})
+
+    response = await handler.create_folder(BadJsonRequest())
+
+    assert response.status == 400
+    payload = json.loads(response.text)
+    assert payload["success"] is False