Add IconButton, Divider, and Link atom components

IconButton:
- Wraps MUI IconButton with FA theme (focus ring, brand hover colours)
- 3 sizes reusing Button height tokens: small 32px, medium 40px, large 48px
- 5 stories: Default, Sizes, Colours, States, CommonUseCases

Divider:
- Wraps MUI Divider with FA border token
- Horizontal/vertical, fullWidth/inset/middle variants, text support
- 6 stories: Default, Variants, Vertical, WithText, InContent, NavigationList

Link:
- Wraps MUI Link with FA brand colour (copper brand.600, 4.8:1 contrast)
- Underline on hover by default, focus-visible ring, fontWeight 500
- 7 stories: Default, UnderlineVariants, ColourVariants, Inline, Navigation,
  FooterLinks, OnDifferentBackgrounds

Theme: Added MuiIconButton, MuiDivider, MuiLink overrides

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/atoms/IconButton/IconButton.tsx

@@ -0,0 +1,40 @@
+import React from 'react';
+import MuiIconButton from '@mui/material/IconButton';
+import type { IconButtonProps as MuiIconButtonProps } from '@mui/material/IconButton';
+
+// ─── Types ───────────────────────────────────────────────────────────────────
+
+/** Props for the FA IconButton component */
+export interface IconButtonProps extends MuiIconButtonProps {}
+
+// ─── Component ───────────────────────────────────────────────────────────────
+
+/**
+ * Icon-only button for the FA design system.
+ *
+ * Square button containing a single icon — used for close buttons, menu
+ * toggles, toolbar actions, and anywhere a text label would be redundant.
+ * Wraps MUI IconButton with FA brand tokens and consistent sizing.
+ *
+ * Sizes use the same height scale as Button:
+ * - `small` — 32px (compact toolbars, card actions)
+ * - `medium` — 40px (default, general actions)
+ * - `large` — 48px (mobile CTAs, meets 44px touch target)
+ *
+ * **Accessibility**: Always provide an `aria-label` prop. Icon-only
+ * buttons have no visible text, so screen readers rely entirely on
+ * the aria-label to announce the action.
+ * ```tsx
+ * <IconButton aria-label="Close dialog">
+ *   <CloseIcon />
+ * </IconButton>
+ * ```
+ */
+export const IconButton = React.forwardRef<HTMLButtonElement, IconButtonProps>(
+  (props, ref) => {
+    return <MuiIconButton ref={ref} {...props} />;
+  },
+);
+
+IconButton.displayName = 'IconButton';
+export default IconButton;