Mobile: choose what swiping a thread row does

[Brechard]

Problem

On mobile, a thread row has one swipe (left), fixed to Settle plus Snooze. People want different actions on that gesture: Archive is missing from the swipe (#9328, #7026), and some would rather swipe to Delete or keep Snooze on its own side. Nothing on the swipe can be changed today.

Proposed behavior

Each side of a thread row gets one action, chosen in Settings → Swipe actions:

• Choices: Settle, Snooze, Archive, Delete, or nothing.
• Defaults: swipe right snoozes, swipe left settles, like swipe actions in mail apps.
• A full swipe runs the action. Snooze only reveals its button, because it needs a wake time and opens the usual time menu.
• On a snoozed thread, the side set to Snooze wakes it. Settle falls back to Archive on servers without settling, and to Un-settle on settled threads.
• Mobile only. The choice is a device preference, so no server or contract change.

One behavior change for existing users: Snooze moves from the left swipe to the right swipe by default.

I have an implementation ready on a branch and will open a PR that links this discussion once the direction is approved.