enhancement: Add Next.js loading.tsx skeleton for country leaderboard pages #276

When navigating between countries in /leaderboard/[country], there is no route-level loading.tsx file. Next.js App Router uses loading.tsx to instantly stream an instant loading state during server rendering.

Motivation

Adding a route-level loading.tsx with skeleton cards prevents perceived lag and layout shift during ISR/SSR country data loading.

Tasks

• Create src/app/leaderboard/[country]/loading.tsx and src/app/leaderboard/loading.tsx.
• Render AppHeader, a table skeleton or cards skeleton matching DashboardSkeleton, and AppFooter.

Files to Check

• src/app/leaderboard/[country]/loading.tsx
• src/app/leaderboard/loading.tsx