feat(i18n): locale-aware formatters and useLanguage hook

- Create useLanguage() hook following useCurrency/useWeightUnit pattern
- Update formatPrice() to use Intl.NumberFormat for locale-aware currency display
- Update formatWeight() to use Intl.NumberFormat for locale-aware number formatting
- Update formatDualPrice() to pass locale through
- Update useFormatters() to pass locale to all formatters
- Add formatter tests for en/de locales (15 tests passing)

Phase 34, Plan 03

tests/formatters.test.ts

@@ -0,0 +1,79 @@
+import { describe, expect, test } from "bun:test";
+import { formatPrice, formatWeight } from "../src/client/lib/formatters";
+
+describe("formatPrice", () => {
+	test("returns -- for null", () => {
+		expect(formatPrice(null)).toBe("--");
+	});
+
+	test("returns -- for undefined", () => {
+		expect(formatPrice(undefined)).toBe("--");
+	});
+
+	test("formats USD with en locale", () => {
+		const result = formatPrice(12345, "USD", "en");
+		expect(result).toContain("123.45");
+		expect(result).toContain("$");
+	});
+
+	test("formats EUR with de locale", () => {
+		const result = formatPrice(12345, "EUR", "de");
+		// German uses comma for decimal
+		expect(result).toContain("123,45");
+	});
+
+	test("formats JPY with no decimals", () => {
+		const result = formatPrice(10000, "JPY", "en");
+		expect(result).toContain("100");
+	});
+
+	test("formats large amounts with en locale", () => {
+		const result = formatPrice(123456789, "USD", "en");
+		expect(result).toContain("$");
+	});
+
+	test("defaults to en locale when no locale provided", () => {
+		const result = formatPrice(12345, "USD");
+		expect(result).toContain("$");
+	});
+});
+
+describe("formatWeight", () => {
+	test("returns -- for null", () => {
+		expect(formatWeight(null)).toBe("--");
+	});
+
+	test("returns -- for undefined", () => {
+		expect(formatWeight(undefined)).toBe("--");
+	});
+
+	test("formats grams with en locale", () => {
+		const result = formatWeight(500, "g", "en");
+		expect(result).toBe("500g");
+	});
+
+	test("formats large grams with en locale thousands separator", () => {
+		const result = formatWeight(1234, "g", "en");
+		expect(result).toContain("g");
+	});
+
+	test("formats ounces", () => {
+		const result = formatWeight(100, "oz", "en");
+		expect(result).toContain("oz");
+	});
+
+	test("formats kilograms", () => {
+		const result = formatWeight(1500, "kg", "en");
+		expect(result).toContain("kg");
+	});
+
+	test("formats pounds", () => {
+		const result = formatWeight(1000, "lb", "en");
+		expect(result).toContain("lb");
+	});
+
+	test("defaults to en locale when no locale provided", () => {
+		const result = formatWeight(500, "g");
+		expect(result).toBe("500g");
+	});
+});