compose: Support editing an already-sent message

[chrisbobbe]

Design from @terpimost:

Details are in Figma:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=4010-6347&m=dev

Updates from the details in Figma:

• For the text when saving the edit is in progress let's say "saving edit…" rather than "sending message…".
• For the text when saving the edit fails let's say "edit wasn't saved" rather than "message wasn't sent".
• Further updates in design thread in chat: #mobile-design > edit message @ 💬

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Original description:

We should probably offer this as an in-place text field within the message list. As Greg points out about this feature in zulip-mobile (link):

In general the edit-message UI in the mobile app is somewhat unloved; as you noted, the UX for it is borrowed from composing a message (and rather awkwardly so), and the code to implement editing a message is likewise rather jammed into the code for composing. One consequence is that it's easy to accidentally pick up behaviors that compose has even when they aren't intended.

[gnprice]

When we implement this, we should aim to offer editing on all message lists, including "All messages" (zulip/zulip-mobile#5619). If we successfully keep at least the code cleanly separated from that for composing a new message, as described above, then that should be straightforward.

(Potentially we'll decide that we want the UX for editing to remain similar to that for composing a message after all. In that case the code will probably have significant pieces in common. But the key will just be to have the differences between edit and compose cleanly factored out, rather than the edit logic being jammed in awkwardly as it is in zulip-mobile.)

[gnprice]

This now has a design — thanks @terpimost! I've edited the issue description to show the design.

Chat thread on a couple of specific points in the design:
https://chat.zulip.org/#narrow/stream/48-mobile/topic/edit-message.20design/near/1930070

I think this is already ripe to work on, though. Both of those are things that will be easy to change later.

Those points have been resolved now; Vlad edited the Figma design and I edited the description above, and together those have the current design that's ready for implementation.