feat: add wallet connect button with address dropdown (Closes #8)

[therealsaitama0]

Summary

Adds a wallet connect button to the top-right header as specified in issue #8.

Changes

• src/components/ui/wallet-button.tsx — New WalletButton component with three states:
  • Disconnected: Shows "Connect Wallet" button → calls useWallet().connect()
  • Connecting: Shows spinner with "Connecting..."
  • Connected: Shows shortened address (e.g. G2AB...K3XP) with green dot indicator
• Connected dropdown (on click): Full address with copy icon (with checkmark feedback), "View on Stellar Explorer" link, and "Disconnect" button (red, clears wallet state)
• src/app/layout.tsx — Added top bar with WalletButton aligned to the right, offset by sidebar width

Acceptance Criteria

• WalletButton component created in src/components/ui/wallet-button.tsx
• All 3 states implemented (disconnected / connecting / connected)
• Address dropdown with copy and explorer link
• Disconnect clears wallet state
• Uses existing useWallet() hook — no reinvention
• Works with Freighter wallet on testnet

Payment

ETHEREUM ADDRESS: 0x5e1040927a1E28D740f92De27a3d493b81682D88