feat: add Shape Pen with live recognition and grid snapping

[devmobasa]

Add Shape Pen so users can draw common shapes without switching between dedicated tools.

Closes #391

• Preview recognized strokes while drawing and commit them as editable lines, circles, ovals, or axis-aligned rectangles. Ambiguous strokes remain freehand.
• Snap lines near Cartesian or isometric board-grid lines. Nearly horizontal or vertical strokes align to that axis; other straight lines retain their angle.
• Add recognition sensitivity from 0 (precise) to 4 (forgiving), defaulting to 2, in configuration and the configurator’s Drawing page.
• Expose Shape Pen through the toolbar’s Shapes picker, command palette, and an optional select_live_shape_tool keybinding.
• Add regression tests for recognition, previews, freehand fallback, grid snapping, sensitivity, and configuration round-tripping.