feat: multi-wallet support (Freighter, xBull, Albedo) - Fixes #66

[harshaaaaw]

Implements #66 - support multiple Stellar wallets beyond Freighter.

Changes:

• src/lib/wallets/types.ts (NEW): WalletAdapter interface.
• src/lib/wallets/freighter.ts (NEW): Freighter adapter via existing @stellar/freighter-api.
• src/lib/wallets/xbull.ts (NEW): xBull adapter via window.xbull global (no extra dep).
• src/lib/wallets/albedo.ts (NEW): Albedo adapter via window.albedo global (no extra dep).
• src/lib/wallets/index.ts (NEW): registry + localStorage key + getAdapter().
• src/app/providers.tsx: selected adapter mgmt, availability detection, remember last-used in localStorage.
• src/components/ConnectWallet.tsx: wallet-selection modal; handles no-wallet-installed case.

Acceptance: adapter interface / 3 adapters / selection modal / localStorage memory / graceful not-installed handling - all met.
xBull+Albedo use injected browser globals so no new npm deps are added (keeps PR mergeable). Old src/lib/wallet.ts left in place (unused) to avoid breaking external imports.

Fixes #66

Payout Address (XMR)

Payout Address: 47SKLyH64Qv63F79iifEuVVvPk1fiqQpY55c6zNLBBZi7xTy239diAZX8oZZmg6231RtaJM6nzfGHdxNnHoTiNS3UV5hPbs