feat(ui): topbar primitives — IconButton, SegmentedControl, Avatar

Adds the shared primitives the planner topbar redesign needs, on the
design-system tokens:

- IconButton — icon-only button (undo/redo), ghost/secondary variants,
  requires an accessible label.
- SegmentedControl — generic single-select toggle (color mode
  Plain/Elevation/Surface), radiogroup semantics.
- Avatar — colored initial circle for participants, per-user color with
  an accent fallback.

Co-located jsdom unit tests for each (roles, aria-checked, onChange,
disabled, color fallback). Extends the dev-only /dev/ui gallery with
sections for all three.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/ui/src/index.ts

@@ -7,3 +7,17 @@ export { Card } from "./Card.tsx";
 export type { CardProps } from "./Card.tsx";
 export { Input } from "./Input.tsx";
 export type { InputProps } from "./Input.tsx";
+export { IconButton } from "./IconButton.tsx";
+export type {
+  IconButtonProps,
+  IconButtonVariant,
+  IconButtonSize,
+} from "./IconButton.tsx";
+export { SegmentedControl } from "./SegmentedControl.tsx";
+export type {
+  SegmentedControlProps,
+  SegmentedOption,
+  SegmentedSize,
+} from "./SegmentedControl.tsx";
+export { Avatar } from "./Avatar.tsx";
+export type { AvatarProps, AvatarSize } from "./Avatar.tsx";