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.