feat(scaler): add ingredient scaling with sensible rounding

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/server/recipes/scaler.ts

@@ -0,0 +1,16 @@
+import type { Ingredient } from '$lib/types';
+
+export function roundQuantity(q: number): number {
+  if (q === 0) return 0;
+  if (q < 1) return Math.round(q * 100) / 100;
+  if (q < 10) return Math.round(q * 10) / 10;
+  return Math.round(q);
+}
+
+export function scaleIngredients(ings: Ingredient[], factor: number): Ingredient[] {
+  if (factor <= 0) throw new Error('factor must be positive');
+  return ings.map((i) => ({
+    ...i,
+    quantity: i.quantity === null ? null : roundQuantity(i.quantity * factor)
+  }));
+}