feat(07-01): implement formatWeight with WeightUnit parameter

- Export WeightUnit type ("g" | "oz" | "lb" | "kg")
- Add conversion constants (GRAMS_PER_OZ, GRAMS_PER_LB, GRAMS_PER_KG)
- Switch-based formatting: g=0dp, oz=1dp, lb=2dp, kg=2dp
- Default parameter "g" preserves backward compatibility
- formatPrice left untouched
- All 21 tests pass

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/client/lib/formatters.ts

@@ -1,6 +1,24 @@
-export function formatWeight(grams: number | null | undefined): string {
+export type WeightUnit = "g" | "oz" | "lb" | "kg";
+
+const GRAMS_PER_OZ = 28.3495;
+const GRAMS_PER_LB = 453.592;
+const GRAMS_PER_KG = 1000;
+
+export function formatWeight(
+	grams: number | null | undefined,
+	unit: WeightUnit = "g",
+): string {
 	if (grams == null) return "--";
-	return `${Math.round(grams)}g`;
+	switch (unit) {
+		case "g":
+			return `${Math.round(grams)}g`;
+		case "oz":
+			return `${(grams / GRAMS_PER_OZ).toFixed(1)} oz`;
+		case "lb":
+			return `${(grams / GRAMS_PER_LB).toFixed(2)} lb`;
+		case "kg":
+			return `${(grams / GRAMS_PER_KG).toFixed(2)} kg`;
+	}
 }
 
 export function formatPrice(cents: number | null | undefined): string {