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/IconButton.tsx

@@ -0,0 +1,50 @@
+import type { ButtonHTMLAttributes } from "react";
+import { cn } from "./cn.ts";
+
+export type IconButtonVariant = "ghost" | "secondary";
+export type IconButtonSize = "sm" | "md";
+
+export interface IconButtonProps
+  extends Omit<ButtonHTMLAttributes<HTMLButtonElement>, "aria-label"> {
+  /** Required accessible name — the button has no visible text. */
+  label: string;
+  variant?: IconButtonVariant;
+  size?: IconButtonSize;
+}
+
+const base =
+  "inline-flex items-center justify-center rounded-md transition-colors " +
+  "focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent " +
+  "focus-visible:ring-offset-1 focus-visible:ring-offset-bg " +
+  "disabled:pointer-events-none disabled:opacity-30";
+
+const variants: Record<IconButtonVariant, string> = {
+  ghost: "text-text-md hover:bg-bg-subtle hover:text-text-hi",
+  secondary:
+    "border border-border bg-bg-raised text-text-hi hover:bg-bg-subtle",
+};
+
+const sizes: Record<IconButtonSize, string> = {
+  sm: "h-7 w-7",
+  md: "h-9 w-9",
+};
+
+export function IconButton({
+  label,
+  variant = "ghost",
+  size = "md",
+  className,
+  type = "button",
+  title,
+  ...props
+}: IconButtonProps) {
+  return (
+    <button
+      type={type}
+      aria-label={label}
+      title={title ?? label}
+      className={cn(base, variants[variant], sizes[size], className)}
+      {...props}
+    />
+  );
+}