Migrate web UI from Streamlit to NiceGUI

Replace the Streamlit-based UI (app.py + tab_*.py) with an event-driven
NiceGUI implementation. This eliminates 135 session_state accesses,
35 st.rerun() calls, and the ui_reset_token hack. Key changes:

- Add main.py as NiceGUI entry point with sidebar, tabs, and file navigation
- Add state.py with AppState dataclass replacing st.session_state
- Add tab_batch_ng.py (batch processor with blur-binding, VACE calc)
- Add tab_timeline_ng.py (history tree with graphviz, batch delete)
- Add tab_raw_ng.py (raw JSON editor)
- Add tab_comfy_ng.py (ComfyUI monitor with polling timer)
- Remove Streamlit dependency from utils.py (st.error → logger.error)
- Remove Streamlit mock from tests/test_utils.py

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_utils.py

@@ -1,15 +1,8 @@
 import json
-import os
 from pathlib import Path
-from unittest.mock import patch
 
 import pytest
 
-# Mock streamlit before importing utils
-import sys
-from unittest.mock import MagicMock
-sys.modules.setdefault("streamlit", MagicMock())
-
 from utils import load_json, save_json, get_file_mtime, ALLOWED_BASE_DIR, DEFAULTS, resolve_path_case_insensitive