feat: add StatsGridSkeleton component for dashboard loading state

[HirenGajjar]

Closes #7

What I did

• Created components/StatsGridSkeleton.tsx with 3 pulsing placeholder cards using Tailwind's animate-pulse
• Used bg-gray-100 / bg-gray-200 for placeholders as specified, with dark mode support
• Integrated into app/page.tsx to show during the loading state before activities are fetched
• No new libraries added — pure Tailwind CSS

Screenshot

[MenachoIvan]

Hey @HirenGajjar! Thanks for the Skeletons, they look great.

Just like in the previous PR, I've noticed this branch is missing the latest updates from dev (including the Empty State logic we just merged).

Could you please sync your branch with dev? You can do this by merging dev into your feature branch or doing a rebase. This will ensure the Skeletons work perfectly with the current dashboard structure and help me test it without conflicts.

Let me know if you need any help with the sync!