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

tasks/ch21-30.md

@@ -156,11 +156,11 @@
 
 ## T23e AI Chat · Welcome
 
-- [ ] WelcomeScreen / Greeting
-- [ ] SuggestionChips / StarterPrompts
-- [ ] PromptLibrary
-- [ ] FollowUpSuggestions
-- [ ] CapabilityCards
+- [x] WelcomeScreen / Greeting → `chat::WelcomeScreen`, a greeting for the hour
+- [x] SuggestionChips / StarterPrompts → `chat::SuggestionChips`
+- [x] PromptLibrary → `documents::TemplatePicker` over saved prompts
+- [x] FollowUpSuggestions
+- [x] CapabilityCards
 
 ## T24 Agent — `src/agent`