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

src/chat/mod.rs

@@ -0,0 +1,26 @@
+mod actions;
+mod code;
+mod list;
+mod media;
+mod message;
+mod play;
+mod status;
+mod stream;
+#[cfg(all(test, feature = "test-support"))]
+mod tests;
+
+pub use actions::{
+    BranchNavigator, MessageActions, MessageEditor, QuoteReply, continue_button, regenerate_button,
+    stop_button,
+};
+pub use code::CodeBlock;
+pub use list::{DateSeparator, MessageList, ScrollToBottomButton};
+pub use media::{FileMessage, ImageGrid, ImageMessage, LinkPreviewCard};
+pub use message::{
+    ChatContainer, MessageAvatar, MessageBubble, MessageFooter, MessageHeader, Role,
+};
+pub use play::{AudioMessage, VideoMessage};
+pub use status::{
+    ErrorMessage, FeedbackForm, RateLimitNotice, ThinkingBlock, ThinkingDuration, ThinkingIndicator,
+};
+pub use stream::{StreamingCursor, StreamingMarkdown, StreamingText};