feat: integrate Freighter wallet connection (Stellar)

[olawaleakanbi035-maker]

closes #386

feat: Setup wallet connection integration (Stellar/Freighter)

Integrates Freighter wallet connectivity to enable users to connect their Stellar wallets for
donations and pool management.

Changes

• Installed @stellar/freighter-api and zustand dependencies
• app/stellar-wallets-kit.ts — Freighter adapter (getPublicKey, connect, disconnect)
• lib/stellar.ts — fetches XLM and USDC balances from Horizon
• components/ConnectWallet.tsx — connect/disconnect button with balance dropdown, error
  handling, and ARIA labels
• components/Navbar.tsx — added ConnectWallet to the header

Behaviour

• "Connect Wallet" button prompts Freighter's requestAccess flow
• Connected state shows shortened address, green indicator, and XLM/USDC balances
• Wallet address persists across page refreshes via zustand persist middleware
• Graceful error message if Freighter is not installed

Testing

• next build passes with no errors or type errors

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