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

examples/gallery/pages/chat/mod.rs

@@ -0,0 +1,45 @@
+mod blocks;
+mod controls;
+mod talk;
+
+use gpui::{AnyElement, App, IntoElement, ParentElement, Window, div};
+
+use super::Page;
+use crate::script::Step;
+
+pub const PAGE: Page = Page {
+    number: 23,
+    slug: "chat",
+    title: "AI Chat",
+    summary: "A conversation with an assistant: messages that stream in, code and media inside them, and what can be done with each.",
+    render,
+    script: SCRIPT,
+};
+
+const SCRIPT: &[Step] = &[
+    Step::DownAt("chat-thinking", 12.0, 10.0),
+    Step::UpAt("chat-thinking", 12.0, 10.0),
+    Step::Wait(300),
+    Step::Shot("reasoning"),
+    Step::DownAt("chat-thinking", 12.0, 10.0),
+    Step::UpAt("chat-thinking", 12.0, 10.0),
+    Step::DownAt("chat-edit-open", 20.0, 12.0),
+    Step::UpAt("chat-edit-open", 20.0, 12.0),
+    Step::Wait(300),
+    Step::Shot("editing"),
+    Step::Key("escape"),
+    Step::Wait(200),
+];
+
+fn render(window: &mut Window, cx: &mut App) -> AnyElement {
+    div()
+        .child(talk::conversation(window, cx))
+        .child(talk::streaming(window, cx))
+        .child(blocks::code(window, cx))
+        .child(blocks::rich(window, cx))
+        .child(blocks::media(window, cx))
+        .child(controls::actions(window, cx))
+        .child(controls::status(window, cx))
+        .child(controls::thinking(window, cx))
+        .into_any_element()
+}