feat: duplicate selected screens with hotspots and connections (Cmd+D)

[trmquang93]

Summary

• Adds duplicateSelection(screenIds) to useScreenManager — deep-clones selected screens with full ID remapping for screen IDs, hotspot IDs, stateGroup, and conditionGroupId; clones only connections where both endpoints are within the selection
• Exposes via Cmd+D keyboard shortcut (blocks browser bookmark dialog with preventDefault)
• Exposes via right-click context menu with smart label: "Duplicate Screen" (single) or "Duplicate Selection" (multi)
• Duplicated screens appear offset +300px horizontally and named with a (copy) suffix
• Post-duplication, the canvas selection switches to the new screens so they can be dragged immediately
• The entire operation is a single undo step via the existing pushHistory pattern

Test plan

• Select 3+ screens with connections between them → Cmd+D → verify new screens appear offset with correct inter-connections
• Hotspots pointing to screens outside the selection still point to the originals (no new external connections created)
• Cmd+Z reverts all duplicated screens and connections in one step
• Right-click single screen (no multi-select) → "Duplicate Screen" works
• Right-click with 2+ screens selected → "Duplicate Selection" works
• isReadOnly mode hides the context menu button and blocks Cmd+D
• Lint and tests pass: npm run lint && npm test