feat(adapters): persist WalletConnect sessions across page reloads

[woahwhattheheck]

Closes #774.

The adapter held the WalletConnect session only in memory, so every page
reload dropped it and the user had to scan a fresh QR code.

Changes

src/adapters/walletconnect.ts:

• Persist on construction. Constructing the adapter is the point at which
  the SDK is handed a live session, so that is where the session is written.
• WalletConnectAdapter.restore(client, storage?, nowSeconds?) — rebuilds
  an adapter from the stored session, or returns null when there is nothing
  usable.
• disconnect() — ends the client session and clears the stored copy.
• Standalone helpers persistWalletConnectSession,
  loadWalletConnectSession, clearWalletConnectSession, and the exported
  WALLETCONNECT_SESSION_KEY, for callers managing the session themselves.

expiry is a Unix timestamp in seconds, matching what WalletConnect puts
on a session object, and is checked on restore.

Details that shape the implementation

• A session with no expiry is not persisted at all. Without one there is
  no way to tell a live session from a dead one on the next load, and silently
  restoring a dead session is worse for the user than asking for a reconnect —
  it fails later, at signing time, instead of immediately.
• disconnect() clears storage in a finally. If the relay is
  unreachable the client's own disconnect rejects; clearing regardless means a
  relay error cannot leave a dead session behind to be restored next load. The
  rejection still propagates.
• A corrupt or expired entry is removed as it is read, so a bad value
  cannot be re-parsed on every subsequent load.
• Every storage access is guarded. localStorage may be absent (Node),
  disabled, full, or — in some privacy modes — a throwing getter rather than
  simply missing. Persisting degrades to a no-op in all of those cases rather
  than breaking an otherwise good session.

Storage is typed structurally (getItem/setItem/removeItem), so a React
Native shim or a test stub can be injected without this module depending on
DOM lib types.

Validation

• npx vitest run test/walletconnect.session.test.ts — 21/21 passing.
  Covers round-trip, namespaced key, expiry (including exactly-at-now),
  unparseable JSON, structurally wrong records, absent store, a store that
  throws on every operation, construction with and without persist,
  construction without expiry, restore hit/expired/empty, restore leaving
  storage untouched, disconnect clearing on success and on relay failure, and
  a client exposing no disconnect.
• Offline: an in-memory store stands in for localStorage and the sign client
  is a stub — no network, no DOM.
• npx tsc --noEmit reports 210 errors on this branch and 210 on unmodified
  main — identical, with none in src/adapters/walletconnect.ts.