feat: add Stellar Freighter wallet connection (#386)

[OpadijoIdris]

Summary

Implement Freighter wallet integration so users can connect, view their
address, and disconnect without leaving the app. All error paths are
handled gracefully with user-friendly messages.

Core additions

• src/context/WalletContext.tsx — React Context provider wrapping
  @stellar/freighter-api. Handles isConnected(), requestAccess(), and
  getAddress() calls. Persists connected address in sessionStorage and
  restores it on mount. Exposes connect(), disconnect(), and
  shortenAddress() via useWallet() hook.

• src/components/wallet/WalletButton.tsx — Navbar button with two states:

  • Disconnected: "Connect Wallet" button with loading spinner.
  • Connected: shortened address (GABC...XYZ1) with dropdown showing
    full address, copy-to-clipboard, Stellar Explorer link, and Disconnect
    option. Error popup auto-dismisses after 6 s; links to freighter.app
    when extension is absent.

Wiring

• src/main.tsx — WalletProvider added inside ThemeProvider/ToastProvider.
• src/components/layout/Navbar.tsx — WalletButton placed between
  ThemeToggle and NotificationCentreDropdown.

Error scenarios handled

• Freighter extension not installed
• User rejects the connection request
• Unexpected network/runtime errors

Tests (18 tests, all passing)

• src/context/__tests__/WalletContext.test.tsx — hook unit tests
• src/components/wallet/__tests__/WalletButton.test.tsx — component tests

Test plan

• User can connect their Freighter wallet
• Connected wallet address is displayed in shortened form in the navbar
• Full address visible in dropdown, with copy and explorer link
• User can disconnect their wallet
• Error shown when Freighter is not installed (with install link)
• Error shown when user rejects the connection request
• Wallet address persists on page refresh within the same session
• All 18 automated tests pass

Closes #386

[vercel]

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[amina69]

@OpadijoIdris , failed