T23c: AI chat, input

- PromptInput: attachments and context above the field, the owner's tools and
  send below; Enter sends, Shift-Enter breaks the line; / offers commands and
  @ offers context over forms::Suggestions; files dropped on it attach behind a
  drop veil
- AttachmentButton, AttachmentChip with thumbnails and uploads, ContextChips
- VoiceInputButton and VoiceWaveform with a red dot and the time
- ParameterPanel, SystemPromptEditor, TokenCounter, CostEstimator
- forms: a Newline action on Shift-Enter keeps one-line fields to one line;
  replace_trigger moves beside Suggestions; choose and dropped shared
- gallery: composer, pickers and tuning

examples/gallery/pages/chat/mod.rs

@@ -1,5 +1,6 @@
 mod blocks;
 mod cite;
+mod compose;
 mod controls;
 mod talk;
 
@@ -53,5 +54,8 @@ fn render(window: &mut Window, cx: &mut App) -> AnyElement {
         .child(controls::thinking(window, cx))
         .child(cite::citations(window, cx))
         .child(cite::search(window, cx))
+        .child(compose::composer(window, cx))
+        .child(compose::pickers(window, cx))
+        .child(compose::tuning(window, cx))
         .into_any_element()
 }