SessionPicker shows ¥ for USD wallets

[esengine]

Follow-up to #272 / #278.

src/cli/ui/SessionPicker.tsx:143-146 still hardcodes ¥ and unconditionally multiplies session cost by USD_TO_CNY, so a USD-wallet user sees ¥X.XX in the session-list picker even after #272 landed:

const costCny =
  info.meta.totalCostUsd !== undefined
    ? `¥${(info.meta.totalCostUsd * USD_TO_CNY).toFixed(2)}`
    : "";

This is the same bug shape as StatusRow / UsageCard, just in a different file. The fix is to swap the literal ¥ + manual conversion for formatCost(info.meta.totalCostUsd, walletCurrency) (the consolidated helper from #272) and thread walletCurrency into the picker — note info.meta only stores totalCostUsd, so the live wallet currency has to come from props/context, not from the session metadata file.

Also need a render test in a new tests/ui-session-picker.test.tsx covering the USD case (assert \$0.05 for totalCostUsd = 0.05 with balanceCurrency = "USD").