T23a: AI chat, messages

- messages: yours in a quiet bubble, the assistant's as prose beside its mark,
  system notes, errors; avatars, headers with model and time, footers
- MessageList over gpui's bottom-aligned list: drawn near the view, held at the
  newest, counting arrivals while away; a way back down; day separators
- streaming text and markdown reveal toward the text as it grows, with a caret;
  a blinking cursor before the first words
- CodeBlock with language, copy, run, apply, download and folding;
  MarkdownRenderer::code lets messages draw fences with it
- pictures, grids, files, voice notes, video over host frames, link previews
- quotes, actions, an editor that takes focus, versions, regenerate, stop and
  continue
- errors with retry, a rate limit counting down, feedback, thinking blocks
- gallery: the AI Chat page with scripted states

TASKS.md

@@ -72,7 +72,7 @@ Per-component lines live in `tasks/`. Tags there:
 - [x] T21b Documents · Reading
 - [x] T21c Documents · Knowledge
 - [x] T22 Collaboration
-- [ ] T23a AI Chat · Messages
+- [x] T23a AI Chat · Messages
 - [ ] T23b AI Chat · Citations
 - [ ] T23c AI Chat · Input
 - [ ] T23d AI Chat · Conversations