Multi-color plotting with pen-change / load-pen pauses

[BernardJen]

Summary

Support multi-color plots where the drawing uses more than one pen. Because the plotter holds a single pen at a time, the plot must pause between color groups so the user can swap (load) the next pen.

Motivation

A single-pen plot can only produce one color. Many drawings (image conversions, multi-layer art) need several colors, which requires manual pen changes mid-plot.

Behavior

• Paths/layers are grouped by pen/color.
• The plot streams all paths for one pen, then pauses and prompts the user to load the next pen.
• After the user confirms the pen is loaded, plotting resumes with the next color group.
• Pen order should be predictable (and ideally reorderable).

Acceptance criteria

• A drawing can have paths assigned to multiple pens/colors.
• Plotting is sequenced per pen: all strokes for pen A, then pen B, etc.
• Between color groups the plot pauses and clearly prompts which pen to load.
• Resuming continues exactly where it left off (no lost/duplicated strokes).
• Preview shows each color group in its pen's color.

Dependencies

• Builds on the pen model from the "Pen selection" issue.

[BernardJen]

Depends on #2 (pen model). Tracked in epic #10.

[BernardJen]

Reopened after a UI review against the acceptance criteria (maintainer's request).

Unmet: "Pen order should be predictable (and ideally reorderable)."

Today the plot order is the pen library's order (penGroups in src/ui/App.tsx iterates pens and keeps whichever have strokes). Since this review the left rail says what that order is — Plots in this order: Black → Red under the pen chip — so it is now predictable. It is still not reorderable: the only way to plot red before black is to reorder the library in Settings → Pens, and there is no control for that either.

Proposed scope

• Drag-to-reorder rows in Settings → Pens, persisted with the app settings on the gateway (src/gateway/appSettings.ts), so every device agrees on the order.
• The order caption in the rail and the pen-change ticks on the transport bar already follow penGroups, so they pick the change up for free.
• No hardware needed to verify; the pure part (generatePenGroupGcode group order) is already covered in src/plot/__tests__/gcode.test.ts — add a case for a non-library order.