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/components/atoms/Card/Card.tsx

@@ -63,10 +63,9 @@ export const Card = React.forwardRef<HTMLDivElement, CardProps>(
         elevation={0}
         sx={[
           // Selected state: brand border + warm background
+          // Border width is always 2px (set in theme) — only colour changes here
           selected && {
             borderColor: 'var(--fa-card-border-selected)',
-            borderWidth: '2px',
-            borderStyle: 'solid',
             backgroundColor: 'var(--fa-card-background-selected)',
           },
           // Interactive: hover fill + shadow lift + pointer