Implement useWallet hook

[iexwr]

Context

src/lib/stellar.ts exposes connectWallet, but there is no React state around it, so components re-implement connection logic.

Acceptance criteria

• Add src/hooks/useWallet.ts returning { publicKey, connected, connect, disconnect, network }.
• connect calls connectWallet() and stores the pubkey; disconnect clears it (and localStorage).
• Persist the connected pubkey across reloads; re-validate Freighter availability on mount.

Technical notes

• Use useState/useEffect; wrap in a context provider in App.tsx if preferred.
• Keep lib/stellar.ts as the low-level signer.

Testing

• Unit test the hook with a mocked stellar-sdk wallet.