DRY: share node-position bounds computation

[lowhung]

Summary

Extract a shared utility to compute bounds for node positions and use it in minimap and SVG export code.

Why

Both draw_minimap and export_to_svg iterate all positions to compute (min_x, max_x, min_y, max_y) with near-identical logic.

Affected files

• src/ui/drawing.rs
• src/ui/export.rs

Extraction plan

• Add utility like positions_bounds(&HashMap<String, Pos2>) -> Option<(f32, f32, f32, f32)>.
• Reuse utility in draw_minimap world-bounds calculation.
• Reuse utility in export_to_svg and keep padding behavior local to export.
• Add tests for empty/non-empty bounds.

Risk

Low. Shared numeric helper; small risk of changing empty-state behavior if not preserved.

Acceptance criteria

• Single bounds utility used by both minimap and SVG export.
• Empty-position behavior unchanged.
• Tests pass.