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

TASKS.md

@@ -74,7 +74,7 @@ Per-component lines live in `tasks/`. Tags there:
 - [x] T22 Collaboration
 - [x] T23a AI Chat · Messages
 - [x] T23b AI Chat · Citations
-- [ ] T23c AI Chat · Input
+- [x] T23c AI Chat · Input
 - [ ] T23d AI Chat · Conversations
 - [ ] T23e AI Chat · Welcome
 - [ ] T24 Agent