WalletModal can crash when no wallet adapters are available

`WalletModal` assumes there is always at least one wallet adapter available when no installed wallets are present.

Evidence in `packages/ui/src/WalletModal.tsx`:
- `getStartedWallet` falls back to `otherWallets[0]!` when there are no installed wallets and the Bread wallet adapter is not found.
- The empty-installed-wallet render path then reads `getStartedWallet.adapter.url` for the Chrome badge link.

If a consumer renders the modal with an empty `wallets` array, or with a set of adapters that does not include the Bread wallet and produces no loadable/not-detected fallback, `getStartedWallet` can be `undefined` at runtime and the modal crashes instead of rendering a stable empty state.

This should be guarded so the modal handles an empty wallet list without throwing.

The issue is in `packages/ui/src/WalletModal.tsx`. Look for the `getStartedWallet` logic and the render path for empty installed wallets. Add a guard to check if `getStartedWallet` is defined before accessing its `adapter.url` property. Test by rendering the modal with an empty `wallets` array to ensure it shows a stable empty state without crashing.

Written by the indexing model from the issue text.