feat: replace Git-DAG timeline with flat snapshot browser

Replace HistoryTree (DAG with branches, Graphviz rendering) with a flat
chronological SnapshotTimeline. New UI features: split-view layout,
snapshot compare/diff, cherry-pick restore of individual sequences or
fields, auto-snapshots with debounce, and pin/filter support.

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

state.py

@@ -13,7 +13,7 @@ class AppState:
     snippets: dict = field(default_factory=dict)
     file_path: Path | None = None
     restored_indicator: str | None = None
-    timeline_selected_nodes: set = field(default_factory=set)
+    timeline_selected_id: str | None = None
     live_toggles: dict = field(default_factory=dict)
     show_comfy_monitor: bool = True