feat: Add wallet connect button to header/navbar

[derekliutao-cmd]

Summary

Closes #8

Implements wallet connect button in the layout header with address display and disconnect.

Changes

src/components/ui/wallet-button.tsx (new file) — WalletButton component:

• Disconnected: "Connect Wallet" button with Wallet icon, calls useWallet().connect()
• Connecting: Spinner with "Connecting..." text
• Connected: Shortened address (e.g. G2AB...K3XP) with animated green ping indicator
• Click when connected → Radix DropdownMenu with:
  • Full address (copyable with copy icon → Check feedback)
  • "View on Stellar Explorer" link (opens testnet explorer)
  • "Disconnect" button (red, calls disconnect())
• Error display when connection fails

src/app/layout.tsx — Updated to render WalletButton:

• Right-aligned in a flex container above page content
• Preserves existing layout structure and metadata

Acceptance Criteria Met

• 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
• Works with Freighter wallet on testnet

Uses existing dependencies (@radix-ui/react-dropdown-menu, lucide-react, clsx, tailwind-merge).