[Feature]: Name a thread when I create it

[equivalent]

Problem

When I start a new thread, I can't give it a name. The name is generated
automatically from my first message. Sometimes I already know what I want
to call the thread, for example because I follow my own naming pattern. Then
I have to wait for the generated name and rename the thread by hand.

With many threads open, my own names make it much easier to find and track
the right thread later.

Proposed solution

Add an optional "Thread name" field to the new-thread screen, next to the
message box.

• If I fill it in, the thread is created with that name, and no name is
  generated.
• If I leave it empty, nothing changes: the name is generated as it is today.

Why this matters

• I can find my threads by the names I chose, not by a summary of the first
  message.
• No extra step after I send the first message, and the generated name never
  shows up only to be replaced.
• No text-generation call for the title when I already gave one.

Smallest useful scope

One optional text field on the new-thread screen on desktop/web. A name typed
there counts as a manual rename, so later title generation leaves it alone.
Mobile can follow later.

Alternatives considered

• Rename the thread after the first message. This works, but it is an extra
  step every time, and the generated name shows up first.
• Custom instructions for title generation. Useful, but it still guesses the
  name. It can't give the exact name I have in mind.

[equivalent]

I opened a draft PR for this: #17621. It adds the optional name field to the new-thread composer on web and desktop, with screenshots in the PR. Happy to change the scope or design if you want it done differently.