feat: highlight last-used wallet provider in ConnectWalletModal (Closes #1287)

[waterWang]

Summary

Adds a "Remember my last used wallet" preference that pre-highlights the last successfully connected wallet provider the next time the ConnectWalletModal opens.

Changes

• src/lib/useLastUsedWallet.ts — New hook + utility functions for reading/writing/clearing the last-used wallet preference in localStorage. Uses useSyncExternalStore for reactive updates across tabs.
• src/components/ConnectWalletModal.tsx — Reads the stored preference on mount; renders a "Last used" badge on the matching wallet option; saves the wallet ID to localStorage on successful Freighter connection.
• src/components/ConnectWalletModal.module.css — Adds .lastUsedBadge style (accent-colored, uppercase, bordered pill).
• src/components/wallet-connect/Walletcontext.tsx — Clears the stored preference on disconnect via clearLastUsedWalletId().
• src/components/__tests__/ConnectWalletModal.test.tsx — 5 new tests covering the preference feature.

Requirements fulfilled

• ✅ Highlighted provider gets a "Last used" badge (not just a color change)
• ✅ Preference is scoped to browser/device (localStorage)
• ✅ Clearing the preference (via disconnect) removes the badge on next open
• ✅ WCAG 2.1 AA accessible (aria-label on badge, works with keyboard nav)
• ✅ Responsive and consistent with existing design tokens