Magnet placement recommendation and visualization

[BernardJen]

Summary

Help the user position the hold-down magnets on the plotter bed: the app both recommends where to place them and visualizes them on the canvas, warning when the plot would collide with a magnet.

Motivation

Magnets keep the paper flat, but a magnet sitting in the pen's travel path will be hit by the pen carriage or smear/snag the drawing. The app can compute safe locations from the drawing's extents and the paper, and show them so the user places magnets confidently.

Behavior

Recommend

• Compute suggested magnet positions based on the paper size and the drawing's bounding box (and/or the pen travel area).
• Prefer corners/edges of the paper that are outside the area the pen will travel.
• Update recommendations as the drawing changes.

Visualize

• Show magnet markers on the canvas at the recommended (and/or user-placed) positions.
• Let the user move/add/remove magnet markers.
• Warn (highlight) when a magnet overlaps the drawing / pen travel area.

Acceptance criteria

• App recommends magnet positions clear of the pen travel / drawing area.
• Recommended magnets are shown on the canvas preview.
• User can reposition / add / remove magnet markers.
• A collision warning appears when a magnet sits within the plot area.
• Recommendations update when the drawing or paper changes.

Notes

• Magnets are preview/advisory only — they do not affect generated plot paths.
• Relates to the paper-type issue (Selectable background paper type #7) since paper size/format drives the safe margins.

[BernardJen]

Advisory overlay, independent — can be built anytime. Relates to #7 (paper size drives safe margins). Tracked in epic #10.