One-handed next / previous email shortcuts (e.g. s / w)

Requested via support by a power user working through large eDiscovery mailboxes, as follow-up polish to the now-shipped z/x/c triage shortcuts.

The up / down arrow keys already move between emails, but they require reaching across the keyboard. For fully one-handed triage alongside z / x / c, it would help to have dedicated letter keys for next / previous — the customer suggested s = next and w = previous, keeping everything under one hand.