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>

state.py

@@ -0,0 +1,17 @@
+from dataclasses import dataclass, field
+from pathlib import Path
+
+
+@dataclass
+class AppState:
+    config: dict
+    current_dir: Path
+    loaded_file: str | None = None
+    last_mtime: float = 0
+    data_cache: dict = field(default_factory=dict)
+    snippets: dict = field(default_factory=dict)
+    file_path: Path | None = None
+    restored_indicator: str | None = None
+    timeline_selected_nodes: set = field(default_factory=set)
+    live_toggles: dict = field(default_factory=dict)
+    show_comfy_monitor: bool = True