Implement wallet connection flow

[Nemenwa]

Implement the wallet connection UI and logic. Support Freighter and xBull wallets.
Components to build:

WalletConnectButton — "Connect Wallet" button in the nav
WalletModal — modal with wallet options
WalletProvider — React context providing wallet state throughout the app
useWallet hook — { address, isConnected, connect, disconnect, signTransaction, signMessage }

Flow:

User clicks "Connect Wallet"
Modal shows: Freighter, xBull, "More coming soon"
User clicks their wallet
Wallet browser extension triggers (if not installed, show install link)
User approves connection
App calls GET /api/auth to get a challenge
App asks wallet to sign the challenge
App submits signed challenge to POST /api/auth to get JWT
JWT stored in an httpOnly cookie (not localStorage)
UI updates to show connected address (truncated: GABCD...WXYZ)