Refresh hub index page card styling (#2400)
* Refresh hub index page card styling
- Softer light-mode gradients (from-*-50/50) and subtle dark-mode tints (dark:from-*-500/5)
- White card titles in dark mode, unified gray-850 borders
- Tighter card spacing (space-y-1) and wider title margin (mb-2)
- Link default text-gray-700, hover text-gray-900; dark:text-gray-400 → dark:hover:text-white
- Drop hover:shadow/transition from container, keep subtle hover translate on links
- not-prose on grid so typography plugin doesn't style the card content
- Refresh Subscriptions & Plans icon (TE) and Agents icon (chibi bot) to match sidebar
* Bump dark-mode card gradients from /5 to /10 for visibility
* Revert light-mode card gradients to original from-*-50
