feat: add memoization, unified search hook, pagination, and SEO meta tags

[bellabuks]

Summary

This PR addresses four performance and SEO improvements:

• Unified search hook (useSearch): Single composable hook with debounce, cursor-based pagination, and load-more support - replaces scattered useAdvancedSearch / useSearchFilters logic
• Memoization: Added useDebounce hook and useMemo to useDashboardData for sorted panels, preventing unnecessary re-renders; also fixed duplicate hook definition bug in the file
• Cursor-based pagination: Updated /api/courses route to accept cursor and limit params and return
  extCursor, and extended PaginatedResponse type accordingly
• SEO meta tags: Added itle, description, openGraph, and witter metadata to the home page, dashboard layout, and profile layout

Changes

• src/hooks/useSearch.tsx - unified search hook with debounce + pagination
• src/hooks/useDebounce.tsx - reusable debounce hook
• src/hooks/useDashboardData.tsx - added useMemo for sorted panels, fixed duplicate content
• src/app/api/courses/route.ts - cursor-based pagination
• src/types/api.ts - added
  extCursor to PaginatedResponse
• src/app/page.tsx - added OpenGraph + Twitter metadata
• src/app/dashboard/layout.tsx - metadata for dashboard route
• src/app/profile/layout.tsx - metadata for profile route

closes #131
closes #133
closes #167
closes #170

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