saveState() is now a read-modify-write that merges only the fields this page
changed, diffed against a deep-cloned per-page baseline. wallets, allowedSites,
deniedSites and networkEndpoints merge structurally by identity, so membership
comes from fresh storage except for this page's own adds and deletes.
Fixes a second extension page silently deleting a wallet, the background balance
refresh clobbering a concurrent add or resurrecting a delete, and a stale page
resurrecting a revoked site permission.
Colliding wallet identities keep both records and log rather than silently
dropping an encryptedSecret. Concurrent writers of the same leaf remain
last-writer-wins by design.
