Refactor for readability: declare state attrs, extract helpers, deduplicate

- Declare dynamic attributes (_render_main, _load_file, etc.) in AppState
  dataclass instead of monkey-patching at runtime
- Extract max_main_seq_number() and FRAME_TO_SKIP_DEFAULT in batch tab
- Add commit() closure in _render_sequence_card to deduplicate save/notify/refresh
- Add default param to dict_number(), replace hand-rolled CFG/VACE/custom bindings
- Extract _delete_nodes() helper in timeline to deduplicate single/batch delete
- Split 230-line render_timeline refreshable into 4 focused section helpers

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

state.py

@@ -1,5 +1,6 @@
 from dataclasses import dataclass, field
 from pathlib import Path
+from typing import Any, Callable
 
 
 @dataclass
@@ -15,3 +16,10 @@ class AppState:
     timeline_selected_nodes: set = field(default_factory=set)
     live_toggles: dict = field(default_factory=dict)
     show_comfy_monitor: bool = True
+
+    # Set at runtime by main.py / tab_comfy_ng.py
+    _render_main: Any = None
+    _load_file: Callable | None = None
+    _main_rendered: bool = False
+    _live_checkboxes: dict = field(default_factory=dict)
+    _live_refreshables: dict = field(default_factory=dict)