DRY: unify edge Bezier control-point calculation

[lowhung]

Summary

Refactor edge curve control point math into a shared helper and reuse it in both runtime drawing and SVG export.

Why

src/ui/drawing.rs and src/ui/export.rs independently compute the same quadratic Bezier control point geometry for edges. This duplication risks drift and subtle visual mismatch.

Affected files

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

Extraction plan

• Add edge_control_point(p0: Pos2, p2: Pos2, curve_cap: f32) -> Pos2 in src/ui/drawing.rs.
• Replace inline control-point math in drawing path with the helper.
• Import and reuse the helper in src/ui/export.rs (with export-specific curve_cap).
• Add/adjust focused unit tests for helper behavior.

Risk

Low. Pure geometry extraction with no behavioral intent change.

Acceptance criteria

• Single helper computes edge control points for both drawing and export paths.
• Existing edge rendering behavior remains visually consistent.
• Tests pass.