feat(sidebar): delete folders from the sidebar (#999)

Folders created from the sidebar had no in-app way back out: the only
removal path was to leave ComfyUI, delete the directory by hand and
rescan. A typo'd folder also polluted the move/download destination
picker permanently, since it reads the same all_folders source.

Adds POST /api/lm/{prefix}/delete-folder, restricted to directories
whose subtree holds no model weight files — a folder-level cascade would
bypass the per-model lifecycle bookkeeping (metadata sidecars, previews,
cache entries, pending-delete staging, recipe references). The service
walks the directory itself instead of trusting the possibly stale cache,
reports what it would remove (models / files / subfolders / symlinks),
and refuses library roots, top-level symlinks (shutil.rmtree rejects
those) and folders holding a staged delete, whose manifest would be
invalidated by the move. Symbolic links inside the subtree are counted
but never followed.

ModelScanner.remove_known_folder mirrors add_known_folder: the removed
subtree leaves all_folders while ancestors are kept (every recorded
ancestor exists on disk in its own right), stale cache entries under the
prefix are purged and the folder list recomputed. The handler broadcasts
models_changed so destination pickers drop the folder too.

The sidebar entry is a destructive context-menu item. The modal opens in
a confirm state for model-free folders and an explanatory one when the
subtree still holds models, decided from the models-only set that already
dims empty nodes; a stale tree is caught by the 409 not_empty/busy
conflict. Truly empty folders get the existing 20s undo affordance,
implemented by re-creating the directory.

tests/services/test_model_scanner.py

@@ -1547,6 +1547,68 @@ async def test_add_known_folder_ignores_empty_input(tmp_path: Path):
     assert cache.all_folders == before
 
 
+@pytest.mark.asyncio
+async def test_remove_known_folder_drops_subtree_and_keeps_ancestors(tmp_path: Path):
+    _create_files(tmp_path)
+    scanner = DummyScanner(tmp_path)
+    await scanner._initialize_cache()
+    cache = await scanner.get_cached_data()
+    await scanner.add_known_folder("nested/deep/leaf")
+
+    await scanner.remove_known_folder("nested/deep")
+
+    assert "nested/deep" not in cache.all_folders
+    assert "nested/deep/leaf" not in cache.all_folders
+    # The ancestor directory still exists on disk in its own right
+    assert "nested" in cache.all_folders
+
+
+@pytest.mark.asyncio
+async def test_remove_known_folder_purges_stale_cache_entries(tmp_path: Path):
+    _, second, _ = _create_files(tmp_path)
+    scanner = DummyScanner(tmp_path)
+    await scanner._initialize_cache()
+    cache = await scanner.get_cached_data()
+    assert "nested" in cache.folders
+
+    await scanner.remove_known_folder("nested")
+
+    assert "nested" not in cache.all_folders
+    assert "nested" not in cache.folders
+    assert _normalize_path(second) not in {
+        item["file_path"] for item in cache.raw_data
+    }
+
+
+@pytest.mark.asyncio
+async def test_remove_known_folder_noop_without_recorded_folders(tmp_path: Path):
+    _create_files(tmp_path)
+    scanner = DummyScanner(tmp_path)
+    await scanner._initialize_cache()
+    cache = await scanner.get_cached_data()
+    cache.all_folders = None
+
+    # Legacy snapshot without recorded folders: nothing to prune, and the
+    # scheduled backfill walk rebuilds the list from disk.
+    await scanner.remove_known_folder("nested")
+
+    assert cache.all_folders is None
+
+
+@pytest.mark.asyncio
+async def test_remove_known_folder_ignores_empty_input(tmp_path: Path):
+    _create_files(tmp_path)
+    scanner = DummyScanner(tmp_path)
+    await scanner._initialize_cache()
+    cache = await scanner.get_cached_data()
+    before = list(cache.all_folders)
+
+    await scanner.remove_known_folder("")
+    await scanner.remove_known_folder("/")
+
+    assert cache.all_folders == before
+
+
 @pytest.mark.asyncio
 async def test_get_all_folders_updated_after_move(tmp_path: Path):
     first, _, _ = _create_files(tmp_path)