feat(33-05): market/currency selector, dual price format, conversion toggle

- Add formatDualPrice() with ~prefix for approximate conversions (D-14)
- Evolve useCurrency() to return CurrencyContext with currency, market, showConversions
- Create useExchangeRates hook + convertClientPrice utility
- Redesign settings: Market & Currency selector, Show Converted Prices toggle
- Add locale-based auto-suggestion banner for first-time currency selection (D-13)
- Update useFormatters to destructure from new CurrencyContext

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/client/lib/formatters.ts

@@ -43,3 +43,23 @@ export function formatPrice(
 	}
 	return `${symbol}${(cents / 100).toFixed(2)}`;
 }
+
+export interface DualPriceOptions {
+	sourceCents: number;
+	sourceCurrency: Currency;
+	targetCurrency: Currency;
+	convertedCents: number;
+}
+
+/**
+ * Format a price with dual display: source price + converted in parentheses.
+ * Per D-14: "€2,000 (~£1,720)" — source prominent, converted approximate.
+ */
+export function formatDualPrice(options: DualPriceOptions): {
+	source: string;
+	converted: string;
+} {
+	const source = formatPrice(options.sourceCents, options.sourceCurrency);
+	const converted = `~${formatPrice(options.convertedCents, options.targetCurrency)}`;
+	return { source, converted };
+}