Add snapshot diff view and bump to v2.4.0

Compare any snapshot vs the current workflow (one click) or two snapshots
against each other (Shift+click to set base). Modal shows added/removed/modified
nodes, widget value changes, property diffs, and rewired connections with
collapsible sections and colored indicators.

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

pyproject.toml

@@ -1,7 +1,7 @@
 [project]
 name = "comfyui-snapshot-manager"
 description = "Automatically snapshots workflow state with a sidebar to browse and restore previous versions."
-version = "2.3.0"
+version = "2.4.0"
 license = {text = "MIT"}
 
 [project.urls]