feat: redesign weight summary legend and add currency selector

Redesign WeightSummaryCard stats from a disconnected 4-column grid to a
compact legend-style list with color dots, percentages, and a divider
before the total row. Switch chart and legend colors to a neutral gray
palette.

Add a currency selector to settings (USD, EUR, GBP, JPY, CAD, AUD) that
changes the displayed symbol across the app. This is visual only — no
value conversion is performed.

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

src/client/hooks/useCurrency.ts

@@ -0,0 +1,12 @@
+import type { Currency } from "../lib/formatters";
+import { useSetting } from "./useSettings";
+
+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";
+}