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/mod.rs

@@ -16,6 +16,7 @@ mod stream;
 #[cfg(all(test, feature = "test-support"))]
 mod tests;
 mod voice;
+mod welcome;
 
 pub use actions::{
     BranchNavigator, MessageActions, MessageEditor, QuoteReply, continue_button, regenerate_button,
@@ -42,3 +43,4 @@ pub use status::{
 };
 pub use stream::{StreamingCursor, StreamingMarkdown, StreamingText};
 pub use voice::{VoiceInputButton, VoiceWaveform};
+pub use welcome::{CapabilityCards, FollowUpSuggestions, SuggestionChips, WelcomeScreen};