Ask which machine before starting "No project" when several machines are connected

[ayoubben18]

Problem

With more than one connected machine, starting a thread without a project still begins on an implicit machine: the current thread's machine on web/desktop, the selected environment on mobile. #14711 asked for a machine choice, and #15356 / #14532 answered it with a machine picker on the draft after it opens.

That picker fixes the dead end, but the first choice is still made for you. The draft opens on the default machine, and that machine's No project folder is set up before you get a chance to change it. The machine control is small and sits below the composer, so it's easy to send on the wrong machine. I've done this when the last thread I touched was on a different box from the one I wanted for throwaway work.

Proposal

Only when more than one connected machine offers No project, the entry points ask instead of defaulting. With one machine, nothing changes.

• Web/desktop: "New thread in…" lists one "No project" row per machine (icon and name, current machine first). "New thread without a project" and ⌥⌘N open a short "Run on" list. "or start without a project" under the draft heading opens a machine menu and keeps any typed prompt.
• Mobile: one "No project · On " row per machine, current machine first.

The draft-page picker from #15356 / #14532 stays as is for switching later.

Prior work

I have a working branch covering web, desktop and iOS, tested with two isolated environments, and can share screenshots. Before opening a PR I'd like to know whether this direction is wanted, since #14532 chose a single No project row on purpose.