T22: Collaboration

- presence: avatars ringed in each peer's color with where they are, press to
  follow; a breathing live dot; follow mode frames the view and stops on Escape
- remote cursors and selections painted over a field from its own layout
- comment threads with replies, reactions and resolve; a marker that opens a
  thread in a bubble; a sidebar of open and resolved threads
- annotations washed in their authors' colors, notes on hover
- suggestion mode: edits become suggestions word by word; track changes shows
  them struck and underlined, to accept or reject
- sharing: invite by email with checked chips, roles, the access list and who
  the link lets in, in one dialog
- primitives::hold_focus gives a mode its own focus, shared with ZenMode
- AvatarGroup's tuck and count shared; Avatar::ring_color; TagInput::check;
  TextInput::bounds_for_range; Popover::size
- icons: smile-plus, user-plus, message-square-diff
- gallery: the Collaboration page with scripted states

TASKS.md

@@ -71,7 +71,7 @@ Per-component lines live in `tasks/`. Tags there:
 - [x] T21a Documents · Editing — `tasks/ch21-30.md`
 - [x] T21b Documents · Reading
 - [x] T21c Documents · Knowledge
-- [ ] T22 Collaboration
+- [x] T22 Collaboration
 - [ ] T23a AI Chat · Messages
 - [ ] T23b AI Chat · Citations
 - [ ] T23c AI Chat · Input