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/hooks/useCurrency.ts

@@ -1,12 +1,42 @@
 import type { Currency } from "../lib/formatters";
 import { useSetting } from "./useSettings";
 
-const VALID_CURRENCIES: Currency[] = ["USD", "EUR", "GBP", "JPY", "CAD", "AUD"];
+const VALID_CURRENCIES: Currency[] = [
+	"USD",
+	"EUR",
+	"GBP",
+	"JPY",
+	"CAD",
+	"AUD",
+];
 
-export function useCurrency(): Currency {
-	const { data } = useSetting("currency");
-	if (data && VALID_CURRENCIES.includes(data as Currency)) {
-		return data as Currency;
-	}
-	return "USD";
+const CURRENCY_MARKET_MAP: Record<string, string> = {
+	EUR: "EU",
+	USD: "US",
+	GBP: "UK",
+	JPY: "JP",
+	CAD: "CA",
+	AUD: "AU",
+};
+
+export interface CurrencyContext {
+	currency: Currency;
+	market: string;
+	showConversions: boolean;
+}
+
+export function useCurrency(): CurrencyContext {
+	const { data: currencyData } = useSetting("currency");
+	const { data: showConversionsData } = useSetting("showConversions");
+
+	const currency: Currency =
+		currencyData && VALID_CURRENCIES.includes(currencyData as Currency)
+			? (currencyData as Currency)
+			: "USD";
+
+	return {
+		currency,
+		market: CURRENCY_MARKET_MAP[currency] ?? currency,
+		showConversions: showConversionsData === "true",
+	};
 }

src/client/hooks/useExchangeRates.ts

@@ -0,0 +1,34 @@
+import { useQuery } from "@tanstack/react-query";
+import { apiGet } from "../lib/api";
+
+interface ExchangeRates {
+	base: string;
+	date: string;
+	rates: Record<string, number>;
+}
+
+export function useExchangeRates() {
+	return useQuery({
+		queryKey: ["exchange-rates"],
+		queryFn: () => apiGet<ExchangeRates>("/api/exchange-rates"),
+		staleTime: 1000 * 60 * 60, // 1 hour client-side stale time
+		gcTime: 1000 * 60 * 60 * 24, // 24 hour garbage collection
+		refetchOnWindowFocus: false,
+	});
+}
+
+/**
+ * Convert price in cents from one currency to another using provided rates.
+ * All conversions go through EUR as the base currency.
+ */
+export function convertClientPrice(
+	cents: number,
+	from: string,
+	to: string,
+	rates: Record<string, number>,
+): number {
+	if (from === to) return cents;
+	const fromRate = rates[from] ?? 1;
+	const toRate = rates[to] ?? 1;
+	return Math.round((cents / fromRate) * toRate);
+}

src/client/hooks/useFormatters.ts

@@ -4,7 +4,7 @@ import { useWeightUnit } from "./useWeightUnit";
 
 export function useFormatters() {
 	const unit = useWeightUnit();
-	const currency = useCurrency();
+	const { currency } = useCurrency();
 	return {
 		weight: (grams: number | null) => formatWeight(grams, unit),
 		price: (cents: number | null) => formatPrice(cents, currency),

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 };
+}

src/client/routes/settings.tsx

@@ -201,11 +201,42 @@ function ImportExportSection() {
 	);
 }
 
+const LOCALE_CURRENCY_MAP: Record<string, Currency> = {
+	de: "EUR",
+	fr: "EUR",
+	es: "EUR",
+	it: "EUR",
+	nl: "EUR",
+	pt: "EUR",
+	en: "USD",
+	ja: "JPY",
+};
+
+function getSuggestedCurrency(): Currency | null {
+	try {
+		const lang = navigator.language;
+		// Check full locale first (e.g., en-GB → GBP)
+		if (lang.startsWith("en-GB")) return "GBP";
+		if (lang.startsWith("en-AU")) return "AUD";
+		if (lang.startsWith("en-CA") || lang.startsWith("fr-CA")) return "CAD";
+		// Fall back to language prefix
+		const prefix = lang.split("-")[0];
+		return LOCALE_CURRENCY_MAP[prefix] ?? null;
+	} catch {
+		return null;
+	}
+}
+
 function SettingsPage() {
 	const unit = useWeightUnit();
-	const currency = useCurrency();
+	const { currency, showConversions } = useCurrency();
 	const updateSetting = useUpdateSetting();
 	const { data: auth } = useAuth();
+	const [suggestionDismissed, setSuggestionDismissed] = useState(false);
+
+	const suggestedCurrency = getSuggestedCurrency();
+	const showSuggestion =
+		!suggestionDismissed && suggestedCurrency && suggestedCurrency !== currency;
 
 	return (
 		<div className="max-w-2xl mx-auto px-4 sm:px-6 lg:px-8 py-6">
@@ -219,6 +250,32 @@ function SettingsPage() {
 				<h1 className="text-xl font-semibold text-gray-900">Settings</h1>
 			</div>
 
+			{showSuggestion && (
+				<div className="bg-blue-50 border border-blue-100 rounded-xl px-4 py-3 mb-4 flex items-center justify-between">
+					<span className="text-sm text-blue-700">
+						Based on your location, we suggest{" "}
+						{CURRENCIES.find((c) => c.value === suggestedCurrency)?.label ??
+							suggestedCurrency}{" "}
+						({suggestedCurrency})
+					</span>
+					<button
+						type="button"
+						onClick={() => {
+							updateSetting.mutate({
+								key: "currency",
+								value: suggestedCurrency,
+							});
+							setSuggestionDismissed(true);
+						}}
+						className="text-sm font-medium text-blue-700 hover:text-blue-800 underline ml-3"
+					>
+						Use{" "}
+						{CURRENCIES.find((c) => c.value === suggestedCurrency)?.label ??
+							suggestedCurrency}
+					</button>
+				</div>
+			)}
+
 			<div className="bg-white rounded-xl border border-gray-100 p-5 space-y-6">
 				<div className="flex items-center justify-between">
 					<div>
@@ -254,10 +311,11 @@ function SettingsPage() {
 
 				<div className="flex items-center justify-between">
 					<div>
-						<h3 className="text-sm font-medium text-gray-900">Currency</h3>
+						<h3 className="text-sm font-medium text-gray-900">
+							Market & Currency
+						</h3>
 						<p className="text-xs text-gray-500 mt-0.5">
-							Changes the currency symbol displayed. This does not convert
-							values.
+							Sets your market region and currency for price display
 						</p>
 					</div>
 					<div className="flex items-center gap-1 bg-gray-100 rounded-full px-1 py-0.5">
@@ -282,6 +340,37 @@ function SettingsPage() {
 						))}
 					</div>
 				</div>
+
+				<div className="border-t border-gray-100" />
+
+				<div className="flex items-center justify-between">
+					<div>
+						<h3 className="text-sm font-medium text-gray-900">
+							Show Converted Prices
+						</h3>
+						<p className="text-xs text-gray-500 mt-0.5">
+							Display approximate conversions when local price is not available
+						</p>
+					</div>
+					<button
+						type="button"
+						onClick={() =>
+							updateSetting.mutate({
+								key: "showConversions",
+								value: showConversions ? "false" : "true",
+							})
+						}
+						className={`relative w-10 h-5 rounded-full transition-colors ${
+							showConversions ? "bg-blue-500" : "bg-gray-200"
+						}`}
+					>
+						<span
+							className={`absolute top-0.5 left-0.5 w-4 h-4 rounded-full bg-white shadow-sm transition-transform ${
+								showConversions ? "translate-x-5" : "translate-x-0"
+							}`}
+						/>
+					</button>
+				</div>
 			</div>
 
 			<div className="bg-white rounded-xl border border-gray-100 p-5 space-y-6 mt-4">