Add snap-to-grid option

[Konstantysz]

Description

Optional grid snapping for cleaner node layouts.

Design

• Toggle: View → Snap to Grid (or hotkey: G)
• Grid size configurable in settings (10px, 20px, 50px)
• Visual grid overlay when enabled (subtle dotted lines)

Behavior

Node positions snap to nearest grid point during drag.

Acceptance Criteria

• Grid size 20px: node at (37, 82) snaps to (40, 80)
• Disable snapping with Shift held during drag