Add Space + drag canvas panning

[panudetjt]

Summary

Adds support for Space + drag canvas panning, a standard interaction pattern used in professional design tools (Figma, Miro,
Adobe products).

Changes

• Added spacePressed state to track Space key
• Added keydown/keyup event listeners to detect Space key with page scroll prevention
• Modified handlePointerDown to initiate panning when Space is held during left-click
• Updated cursor feedback: "grab" when Space is held, "grabbing" during active pan

Behavior

Action	Result
Hold Space	Cursor changes to "grab"
Space + left-click drag	Canvas pans with "grabbing" cursor
Release mouse (Space still held)	Cursor returns to "grab"
Release Space	Cursor returns to "default"
Space + click on element	Pans canvas (does not select element)

Compatibility

• Existing middle-mouse panning still works
• Normal left-click selection/dragging unchanged
• Mouse wheel panning unchanged

Test plan

• Hold Space key shows "grab" cursor
• Space + left-click drag pans the canvas
• Normal left-click still selects/drags elements
• Middle-mouse button still works for panning
• Space doesn't scroll the page when canvas is focused

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