Extract the balance store's viewport/batch/cancel/cache scheduler into a
generic createViewportStore (wative/viewport), parameterised by the loaded
payload. Reimplement createBalanceStore on it (public API unchanged — its
tests are the regression guard) and add createNonceStore (wative/nonces) for
a wallet list's pending-nonce column: same batching, cursor-crawl,
cancel-on-scroll and caching; EVM-only, off-network wallets load as na.

Add the cancellable single read readWalletNonce (AbortSignal) in session.ts,
mirroring readWalletBalances.

Bump 2.1.0 -> 2.1.1 (additive, non-breaking).