feat: scaffold Svelte app with shared types and scale calculation

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

tests/scale.test.ts

@@ -0,0 +1,18 @@
+import { describe, expect, it } from 'vitest';
+import { scaleFromLine } from '../src/lib/scale';
+
+describe('scaleFromLine', () => {
+  it('returns pixels per mm', () => {
+    expect(scaleFromLine({ x: 0, y: 0 }, { x: 300, y: 400 }, 100)).toBeCloseTo(5);
+  });
+
+  it('rejects a non-positive or missing length', () => {
+    expect(scaleFromLine({ x: 0, y: 0 }, { x: 100, y: 0 }, 0)).toBeNull();
+    expect(scaleFromLine({ x: 0, y: 0 }, { x: 100, y: 0 }, -5)).toBeNull();
+    expect(scaleFromLine({ x: 0, y: 0 }, { x: 100, y: 0 }, NaN)).toBeNull();
+  });
+
+  it('rejects a line shorter than one pixel', () => {
+    expect(scaleFromLine({ x: 10, y: 10 }, { x: 10.5, y: 10 }, 100)).toBeNull();
+  });
+});