acp: Animate loading context creases (#36814)

- Add pulsating animation for context creases while they're loading
- Add spinner in message editors (replacing send button) during the
window where sending has been requested, but we haven't finished loading
the message contents to send to the model
- During the same window, ignore further send requests, so we don't end
up sending the same message twice if you mash enter while loading is in
progress
- Wait for context to load before rewinding the thread when sending an
edited past message, avoiding an empty-looking state during the same
window

Release Notes:

- N/A

Cargo.lock

@@ -403,6 +403,7 @@ dependencies = [
  "parking_lot",
  "paths",
  "picker",
+ "postage",
  "pretty_assertions",
  "project",
  "prompt_store",