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/services/test_model_file_service.py

@@ -0,0 +1,89 @@
+"""Tests for ModelMoveService folder creation."""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import List
+
+import pytest
+
+from py.services.model_file_service import ModelMoveService
+
+
+class FakeScanner:
+    def __init__(self, roots: List[Path]) -> None:
+        self._roots = [str(root) for root in roots]
+        self.known_folders: List[str] = []
+
+    def get_model_roots(self) -> List[str]:
+        return list(self._roots)
+
+    async def add_known_folder(self, folder: str) -> None:
+        self.known_folders.append(folder)
+
+
+@pytest.mark.asyncio
+async def test_create_folder_creates_directory_and_registers_it(tmp_path: Path):
+    scanner = FakeScanner([tmp_path])
+    service = ModelMoveService(scanner, "lora")
+
+    target = tmp_path / "characters" / "anime"
+    result = await service.create_folder(str(target))
+
+    assert result["success"] is True
+    assert result["created"] is True
+    assert result["folder"] == "characters/anime"
+    assert target.is_dir()
+    assert scanner.known_folders == ["characters/anime"]
+
+
+@pytest.mark.asyncio
+async def test_create_folder_existing_directory_reports_not_created(tmp_path: Path):
+    (tmp_path / "existing").mkdir()
+    scanner = FakeScanner([tmp_path])
+    service = ModelMoveService(scanner, "lora")
+
+    result = await service.create_folder(str(tmp_path / "existing"))
+
+    assert result["success"] is True
+    assert result["created"] is False
+    assert result["folder"] == "existing"
+
+
+@pytest.mark.asyncio
+async def test_create_folder_rejects_paths_outside_roots(tmp_path: Path):
+    root = tmp_path / "library"
+    root.mkdir()
+    scanner = FakeScanner([root])
+    service = ModelMoveService(scanner, "lora")
+
+    outside = tmp_path / "outside"
+    result = await service.create_folder(str(outside))
+
+    assert result["success"] is False
+    assert "error" in result
+    assert not outside.exists()
+    assert scanner.known_folders == []
+
+
+@pytest.mark.asyncio
+async def test_create_folder_rejects_traversal_outside_roots(tmp_path: Path):
+    root = tmp_path / "library"
+    root.mkdir()
+    scanner = FakeScanner([root])
+    service = ModelMoveService(scanner, "lora")
+
+    result = await service.create_folder(str(root / ".." / "escape"))
+
+    assert result["success"] is False
+    assert not (tmp_path / "escape").exists()
+    assert scanner.known_folders == []
+
+
+@pytest.mark.asyncio
+async def test_create_folder_requires_path(tmp_path: Path):
+    service = ModelMoveService(FakeScanner([tmp_path]), "lora")
+
+    result = await service.create_folder("")
+
+    assert result["success"] is False