feat(ui): show session cost in VS Code context usage readout (#2991)

Share the desktop session-card currency formatter (lib/money.ts) and surface
the current session's cost in the extension chat-header context usage tooltip.

packages/ui/src/lib/money.ts

@@ -0,0 +1,19 @@
+import { getCurrentIntlLocale } from './i18n';
+
+
+export const formatMoney = (value: number): string => {
+  if (!Number.isFinite(value) || value <= 0) {
+    return new Intl.NumberFormat(getCurrentIntlLocale(), {
+      style: 'currency',
+      currency: 'USD',
+      minimumFractionDigits: 2,
+      maximumFractionDigits: 2,
+    }).format(0);
+  }
+  return new Intl.NumberFormat(getCurrentIntlLocale(), {
+    style: 'currency',
+    currency: 'USD',
+    minimumFractionDigits: value < 0.01 ? 4 : 2,
+    maximumFractionDigits: value < 0.01 ? 4 : 2,
+  }).format(value);
+};