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/theme/index.ts

@@ -679,6 +679,68 @@ export const theme = createTheme({
         },
       },
     },
+    MuiIconButton: {
+      styleOverrides: {
+        root: {
+          borderRadius: parseInt(t.ButtonBorderRadiusDefault, 10),
+          transition: 'background-color 150ms ease-in-out, box-shadow 150ms ease-in-out',
+          '&:focus-visible': {
+            outline: `2px solid ${t.ColorInteractiveFocus}`,
+            outlineOffset: '2px',
+          },
+        },
+        sizeSmall: {
+          width: parseInt(t.ButtonHeightSm, 10),
+          height: parseInt(t.ButtonHeightSm, 10),
+          '& .MuiSvgIcon-root': { fontSize: t.ButtonIconSizeSm },
+        },
+        sizeMedium: {
+          width: parseInt(t.ButtonHeightMd, 10),
+          height: parseInt(t.ButtonHeightMd, 10),
+          '& .MuiSvgIcon-root': { fontSize: t.ButtonIconSizeMd },
+        },
+        sizeLarge: {
+          width: parseInt(t.ButtonHeightLg, 10),
+          height: parseInt(t.ButtonHeightLg, 10),
+          '& .MuiSvgIcon-root': { fontSize: t.ButtonIconSizeLg },
+        },
+        colorPrimary: {
+          color: t.ColorInteractiveDefault,
+          '&:hover': { backgroundColor: t.ColorBrand100 },
+        },
+        colorSecondary: {
+          color: t.ColorNeutral600,
+          '&:hover': { backgroundColor: t.ColorNeutral200 },
+        },
+      },
+    },
+    MuiDivider: {
+      styleOverrides: {
+        root: {
+          borderColor: t.ColorBorderDefault,
+        },
+      },
+    },
+    MuiLink: {
+      defaultProps: {
+        underline: 'hover',
+      },
+      styleOverrides: {
+        root: {
+          color: t.ColorTextBrand,
+          fontWeight: 500,
+          transition: 'color 150ms ease-in-out',
+          '&:hover': {
+            color: t.ColorInteractiveActive,
+          },
+          '&:focus-visible': {
+            outline: `2px solid ${t.ColorInteractiveFocus}`,
+            outlineOffset: '2px',
+            borderRadius: '2px',
+          },
+        },
+      },
+    },
     MuiRadio: {
       styleOverrides: {
         root: {