Selectable background paper type

[BernardJen]

Summary

Allow the user to change the background paper shown behind the drawing to a different paper type.

Motivation

The canvas background currently represents a single paper. Letting the user pick a paper type makes the preview match the real medium and helps visualize how a drawing will look on different stock (e.g. tone/color, grid/dot/lined).

Paper characteristics (TBD during design)

• Paper type / texture: plain, dotted, grid, lined, etc.
• Color / tone: white, off-white/cream, colored stock, dark (for white/metallic pens).
• (Optional) size/format already handled elsewhere — this issue is about appearance/type.

Acceptance criteria

• User can select among multiple background paper types.
• The canvas preview updates to reflect the chosen paper.
• The selected paper persists with the session.
• Paper background is preview-only and does not affect generated plot paths.

[BernardJen]

Depends on #6 (object/scene model). Tracked in epic #10.