feat: link existing screens as state variants via drag

[trmquang93]

Summary

• When dragging a connection between two screens, a popup now asks "Navigate" or "State Variant" — similar to the existing conditional branch prompt
• Choosing State Variant links the target screen into the source screen's state group using the existing stateGroup/stateName system
• New linkAsState(screenId, parentScreenId) function in useScreenManager handles the grouping logic with proper undo support
• Clicking away from the prompt defaults to "Navigate" (preserves existing drag-to-connect behavior)

Test plan

• Drag a connection from screen A to screen B — verify the "Connection type?" popup appears
• Click "Navigate" — verify a normal navigation connection is created
• Drag again and click "State Variant" — verify both screens join the same state group with dashed connector
• Verify sidebar shows sibling states when selecting a linked screen
• Verify undo (Ctrl+Z) reverses the state link
• Verify existing conditional prompt still works (drag a second connection from a screen that already has one)
• npm test passes (281 tests, including 7 new linkAsState tests)
• npm run build succeeds