v1.7.0: graph diff view

What changed

• Added a Diff toggle for comparing the original graph with the active edited state.
• Deleted nodes return as non-interactive ghost cards in the comparison overlay.
• Existing amber MOD and green NEW badges identify modified and added nodes.
• Changed current connections render in blue, while removed original connections render as dashed red paths.
• Added a Changes sidebar tab with a compact, copyable plain-text change log.
• The overlay and change log follow the active history point, including undo, redo, and direct history jumps.

Verification

• 281 tests passed across 19 files.
• Lint and production build completed successfully.
• Verified in Chromium by loading an ONNX model, adding a node, enabling Diff, and checking the change log.