fix(ui): add isDark prop to Skeleton component and resolve type errors (closes #1081)

[rupesh-kumar-sah]

Summary

Closes #1081

Problem

The shared UI skeleton component src/components/ui/Skeleton.tsx declared SkeletonProps without an isDark field. However, skeleton screens such as src/components/mobile/TeamDashboard/DashboardSkeleton.tsx and src/components/mobile/CourseViewerSkeleton.tsx pass isDark to <UISkeleton ... isDark={isDark} />, causing TypeScript errors (Property 'isDark' does not exist on type 'IntrinsicAttributes & SkeletonProps').

Solution

1. Added isDark?: boolean to SkeletonProps in src/components/ui/Skeleton.tsx.
2. Applied theme-aware placeholder background styling:
   • isDark ? '#334155' : '#E5E7EB' consistent with design tokens across src/components/common/SkeletonLoader.tsx and DashboardSkeleton.tsx.
3. Cleaned up imports: Removed unused StyleSheet import from src/components/ui/Skeleton.tsx.
4. Enhanced Unit Tests: Added comprehensive test cases to tests/components/Skeleton.test.tsx validating dark mode styling when isDark=true and default light styling when isDark is false or omitted.

Verification

• Unit Tests: npm test tests/components/Skeleton.test.tsx passed (16/16 tests green).
• ESLint: npx eslint src/components/ui/Skeleton.tsx tests/components/Skeleton.test.tsx (0 errors, 0 warnings).
• Prettier: Verified full code formatting compliance.
• TypeScript: Verified zero type errors on Skeleton and consuming dashboard components.