Draw basic shapes and text on the canvas

[BernardJen]

Summary

Add tools to draw basic shapes and text directly on the canvas, producing plottable paths.

Motivation

Not every plot starts from an imported image. Users should be able to compose drawings natively with primitives and text.

Shapes

• Line
• Rectangle / square
• Ellipse / circle
• Polygon / polyline
• (Optional) freehand path

Text

• Place text on the canvas.
• Choose a (plotter-friendly / single-line) font.
• Adjust size and position.
• Text is rendered as plottable strokes/paths (not filled glyphs).

Acceptance criteria

• User can add each basic shape and position/resize it.
• User can add text, set its content, font, and size.
• Shapes and text become plottable paths included in the output.
• Items render correctly in the canvas preview.

Dependencies

• Drawing/placement tools likely live alongside the toolbar (see the toolbar issue).

[BernardJen]

Depends on #6 (object/scene model). Note: plotter text needs single-line / Hershey-style fonts, not outline fonts. Tracked in epic #10.