useWallet's missing-context guard is unreachable

[luchobonatti]

Description

useWallet at app/src/hooks/useWallet.ts:6 throws when the wallet context is
falsy, but WalletProvider at app/src/providers/WalletProvider.tsx:40
creates the context with a real default object (behind a @ts-ignore), so
that throw branch can never execute.

Steps to reproduce

1. Render any component that calls useWallet() without wrapping it in
   WalletProvider.
2. Observe that it returns the default context object instead of throwing.

Expected vs actual behavior

Expected: calling useWallet() outside a WalletProvider throws, since
the hook exists to guard against exactly that.
Actual: it silently returns the default context object, so the guard
never fires.

Additional context

app/src/components/NetworkPill.test.tsx depends on exactly this behavior —
it renders NetworkPill with no WalletProvider and asserts the
deterministic disconnected state that only works because useWallet()
doesn't throw. Fixing the guard to actually throw will turn that test red;
whoever picks this up needs to wrap that test's render in WalletProvider or
a stub context, which is a two-line change, not a redesign.