enhancement: snap previewed lines and shapes to common angles and nearby geometry

[markm39]

Problem

Previewed shapes and lines should be easier to align while the user is still holding the pencil. Currently, recognized previews do not snap strongly enough to common construction angles or nearby geometry.

Expected behavior

While a line or shape preview is active, the preview should optionally snap to useful geometry:

• Common angles such as 0, 45, and 90 degrees.
• Horizontal/vertical alignment.
• Parallel or orthogonal alignment to nearby lines.
• Endpoint snapping to nearby shape/line endpoints where appropriate.

Notes

This should feel like a drawing-assist feature, not a fight against the user. Snapping should have sensible thresholds and should be easy to escape by moving farther away from the snap target.

Acceptance criteria

• Previewed lines snap to common angles.
• Previewed shapes can align horizontally/vertically where appropriate.
• Snapping thresholds feel predictable and do not interfere with freehand drawing.
• Existing shape recognition still works.