Add shadcn/ui design system, theme, and base components

Install shadcn/ui with base-nova style, tailwind-merge, tw-animate-css,
and font packages. Add oklch-based light/dark theme in index.css and
17 base UI components (button, card, dialog, table, tabs, etc.) plus
shared lib utilities (format, colors, cn) and useSettings hook.

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

frontend/src/lib/format.ts

@@ -0,0 +1,13 @@
+export function formatAmount(amount: number, currency: string) {
+  const abs = Math.abs(amount);
+  if (currency === 'BTC') return abs.toFixed(8);
+  if (currency === 'XMR') return abs.toFixed(4);
+  if (currency === 'USD') {
+    return `$${abs.toLocaleString('en-US', { minimumFractionDigits: 2, maximumFractionDigits: 2 })}`;
+  }
+  return `₡${abs.toLocaleString('es-CR', { minimumFractionDigits: 2, maximumFractionDigits: 2 })}`;
+}
+
+export function formatDate(dateStr: string) {
+  return new Date(dateStr).toLocaleDateString('en-US', { month: 'short', day: 'numeric' });
+}