feat: scaffold Svelte app with shared types and scale calculation

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/lib/scale.ts

@@ -0,0 +1,7 @@
+import type { Point } from './types';
+
+export function scaleFromLine(a: Point, b: Point, lengthMm: number): number | null {
+  const lengthPx = Math.hypot(b.x - a.x, b.y - a.y);
+  if (!(lengthMm > 0) || lengthPx < 1) return null;
+  return lengthPx / lengthMm;
+}