Fix Card selection layout shift and add transitions

- Reserve 2px border on all cards (transparent for elevated, coloured
  for outlined) so selected state only changes colour, not width
- Add 150ms ease-in-out transitions for border-color and background-color
  consistent with Button/Input interactive timing
- Remove borderWidth/borderStyle from selected sx (now handled by theme)

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

src/theme/index.ts

@@ -479,7 +479,10 @@ export const theme = createTheme({
           borderRadius: parseInt(t.CardBorderRadiusDefault, 10),
           backgroundColor: t.CardBackgroundDefault,
           boxShadow: t.CardShadowDefault,
-          transition: 'box-shadow 150ms ease-in-out',
+          // Reserve 2px border on ALL cards (transparent for elevated, coloured for outlined).
+          // Prevents layout shift when toggling selected state.
+          border: '2px solid transparent',
+          transition: 'box-shadow 150ms ease-in-out, border-color 150ms ease-in-out, background-color 150ms ease-in-out',
         },
       },
       variants: [