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>

utils.py

@@ -5,8 +5,6 @@ import time
 from pathlib import Path
 from typing import Any
 
-import streamlit as st
-
 # --- Magic String Keys ---
 KEY_BATCH_DATA = "batch_data"
 KEY_HISTORY_TREE = "history_tree"
@@ -145,7 +143,7 @@ def load_json(path: str | Path) -> tuple[dict[str, Any], float]:
             data = json.load(f)
         return data, path.stat().st_mtime
     except Exception as e:
-        st.error(f"Error loading JSON: {e}")
+        logger.error(f"Error loading JSON: {e}")
         return DEFAULTS.copy(), 0
 
 def save_json(path: str | Path, data: dict[str, Any]) -> None: