Persist wallet session across page reloads

[Dannyswiss1]

Currently reloading the page re-runs useWallet's isConnected/isAllowed check, which works, but there's no
optimistic "restoring session…" state — the wallet gate briefly flashes before resolving. Add a short-lived
loading state distinct from unavailable/idle that renders while the initial Freighter check is in flight, so
users with an already-approved wallet don't see a false disconnect flash.

Acceptance criteria

• Initial load shows a distinct "restoring session" state, not the full disconnected gate, while isConnected/
  isAllowed resolve
• No visible flash of the disconnected state for already-approved wallets on reload
• Covered by a test added to Unit tests for the mock API layer (client/src/lib/api.ts) #5