Implement Freighter Wallet Integration

[Luluameh]

Source: FRONTEND_ISSUES.md (Issue 9)

Background
Users need to connect their Stellar/Soroban wallets to interact with the platform.
Goal
Integrate @stellar/freighter-api to enable wallet connection and signature requests.
Acceptance Criteria

• "Connect Wallet" button on the Navbar.
• Displays connected address and balance.
• Handles wallet disconnect and network change events.
  Key Files
  src/components/layout/Navbar.tsx, src/providers/WalletProvider.tsx

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