T23e: AI chat, welcome

- WelcomeScreen: a greeting for the hour over the owner's composer,
  starters and cards, centered while it fits and scrolled from its top
  when it does not
- SuggestionChips, FollowUpSuggestions and CapabilityCards: each press asks
  the owner; rows and cards are Tab stops
- PromptLibrary is documents::TemplatePicker over saved prompts
- gallery: starters and cards fill the welcome composer, a saved prompt
  fills it too, send clears it with a toast, a follow-up asks its question

src/chat/tests/mod.rs

@@ -10,6 +10,7 @@ use gpui::{
 mod cite;
 mod compose;
 mod history;
+mod welcome;
 
 use super::{
     ImageMessage, MessageAvatar, MessageBubble, MessageEditor, MessageList, Role, ThinkingBlock,