feat: add shift and alt/option modifiers for shape drawing (#1177)

[Pablo31Monster]

Summary

Adds keyboard modifiers for drawing shapes by dragging on the canvas, so users can control both a shape’s proportions and its anchor point while creating it.

Changes

• Shift constrains proportions: rectangles become squares, ellipses become circles, and polygons follow the proportions supported by their tool.
• Alt/Option uses the initial click as the shape’s center, drawing outward as the pointer moves.
• Shift + Alt/Option combines centered drawing with constrained proportions.
• Applies these behaviors to the Rectangle, Ellipse, and Polygon tools.
• Updates the relevant documentation and changelog entry for issue Feature request: Shift and Alt/Option modifiers when drawing shapes (proportions and center-based drawing) #1177.

Validation

• SDK tests pass.
• SDK build succeeds.
• Manual validation in localhost

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