fix dark/light mode surface colors and Remove button color

- Fix ContentViewer background invisible against card in dark mode
- Fix Remove button not red in dark mode by using color="red.6"
- Add distinct card background for light mode (gray-0) vs app background
- Use CSS custom properties for card background across color schemes

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

clients/web/src/components/molecules/ServerCard/ServerCard.tsx

@@ -117,7 +117,7 @@ export function ServerCard({
             <Button variant="subtle" size="xs" onClick={onEdit}>
               Edit
             </Button>
-            <Button variant="transparent" size="xs" color="red" onClick={onRemove}>
+            <Button variant="subtle" size="xs" color="red.6" onClick={onRemove}>
               Remove
             </Button>
           </ActionsRow>