test(07-01): add failing tests for formatWeight unit conversion

- Tests for all 4 units (g, oz, lb, kg) with known gram values
- Null and undefined handling for each unit
- Default parameter backward compatibility
- Zero and precision edge cases
- 11 pass (existing behavior), 10 fail (new unit conversion)

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

tests/lib/formatters.test.ts

@@ -0,0 +1,98 @@
+import { describe, expect, test } from "bun:test";
+import { formatWeight } from "@/client/lib/formatters";
+
+describe("formatWeight", () => {
+	describe("grams (default)", () => {
+		test("formats grams with no decimal", () => {
+			expect(formatWeight(100, "g")).toBe("100g");
+		});
+
+		test("rounds fractional grams", () => {
+			expect(formatWeight(99.6, "g")).toBe("100g");
+		});
+
+		test("formats zero grams", () => {
+			expect(formatWeight(0, "g")).toBe("0g");
+		});
+
+		test("defaults to grams when no unit specified (backward compatible)", () => {
+			expect(formatWeight(100)).toBe("100g");
+		});
+	});
+
+	describe("ounces", () => {
+		test("converts grams to ounces with 1 decimal", () => {
+			expect(formatWeight(100, "oz")).toBe("3.5 oz");
+		});
+
+		test("converts 1000g to ounces", () => {
+			expect(formatWeight(1000, "oz")).toBe("35.3 oz");
+		});
+
+		test("formats zero as ounces", () => {
+			expect(formatWeight(0, "oz")).toBe("0.0 oz");
+		});
+	});
+
+	describe("pounds", () => {
+		test("converts grams to pounds with 2 decimals", () => {
+			expect(formatWeight(1000, "lb")).toBe("2.20 lb");
+		});
+
+		test("handles small weight in pounds", () => {
+			expect(formatWeight(5, "lb")).toBe("0.01 lb");
+		});
+
+		test("formats zero as pounds", () => {
+			expect(formatWeight(0, "lb")).toBe("0.00 lb");
+		});
+	});
+
+	describe("kilograms", () => {
+		test("converts grams to kilograms with 2 decimals", () => {
+			expect(formatWeight(1500, "kg")).toBe("1.50 kg");
+		});
+
+		test("handles large weight in kilograms", () => {
+			expect(formatWeight(50000, "kg")).toBe("50.00 kg");
+		});
+
+		test("converts 1000g to 1 kg", () => {
+			expect(formatWeight(1000, "kg")).toBe("1.00 kg");
+		});
+
+		test("formats zero as kilograms", () => {
+			expect(formatWeight(0, "kg")).toBe("0.00 kg");
+		});
+	});
+
+	describe("null and undefined handling", () => {
+		test("returns '--' for null with no unit", () => {
+			expect(formatWeight(null)).toBe("--");
+		});
+
+		test("returns '--' for null with grams", () => {
+			expect(formatWeight(null, "g")).toBe("--");
+		});
+
+		test("returns '--' for null with ounces", () => {
+			expect(formatWeight(null, "oz")).toBe("--");
+		});
+
+		test("returns '--' for null with pounds", () => {
+			expect(formatWeight(null, "lb")).toBe("--");
+		});
+
+		test("returns '--' for null with kilograms", () => {
+			expect(formatWeight(null, "kg")).toBe("--");
+		});
+
+		test("returns '--' for undefined with kilograms", () => {
+			expect(formatWeight(undefined, "kg")).toBe("--");
+		});
+
+		test("returns '--' for undefined with no unit", () => {
+			expect(formatWeight(undefined)).toBe("--");
+		});
+	});
+});