[GF-09] Add Freighter wallet integration with multi-wallet support and session persistence

[Leothosine]

Summary

Implements wallet integration as described in #42 - adds Freighter wallet connectivity, a multi-wallet picker (Freighter, LOBSTR, xBull), and 24-hour session persistence via sessionStorage.

Changes

• �pp/src/lib/wallet/types.ts - shared types: WalletProvider, StellarNetwork, WalletAccount, WalletSession, WalletAdapter, SignTransactionOptions
• �pp/src/lib/wallet/session.ts - session persistence helpers (saveSession, loadSession, clearSession, createSession) with TTL-based expiry
• �pp/src/components/wallet/WalletButton.tsx - client component with wallet picker dropdown, Freighter connection flow, and network detection

Behaviour

• On load, the component restores any existing session from sessionStorage
• Clicking Connect Wallet opens a provider picker (Freighter, LOBSTR, xBull)
• Freighter path calls
  equestAccess() and getNetwork() to detect mainnet/testnet/futurenet automatically
• LOBSTR and xBull show a ''coming soon'' message (stubs for follow-up PRs)
• Session expires after 24 hours; expired sessions are cleared automatically on next load
• Disconnecting clears both state and storage

closes #42

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