Fix: Don't cancel bounding box drawing when mouse leaves canvas

[iwconfig]

When drawing a bounding box, if the cursor leaves the canvas area, the selection is no longer cancelled. Instead:

• The box edge is clamped to canvas boundary while outside
• Mouse position is tracked globally while outside
• Box creation completes normally when mouse re-enters and releases
• If released outside canvas, box is created with edge at canvas boundary

Implementation details:

• Uses isOutside state to track when cursor is outside canvas
• Global event handlers only attached when both drawing AND outside canvas
• addingRef provides synchronous access to adding state in event handlers
• Immediate ref update in createBoundingBox prevents race conditions

This improves UX when drawing large boxes that may require the cursor to briefly exit the canvas area.