test: Add comprehensive Courses page tests

[DSOTec]

Summary

Adds 12 comprehensive tests for the Courses page (src/pages/Courses.tsx) covering filtering, search, pagination, loading, and empty states.

Closes #632

Type of Change

• Bug fix
• New feature
• Smart contract change
• Documentation
• Breaking change
• Other (please describe) — Test coverage

Checklist

• Tests added or updated
• No hardcoded secrets or private keys
• If contract change: cargo test passes
• If frontend change: npm run typecheck passes
• If backend change: npm test passes
• Related issues linked (Closes feat: write tests for Courses page (filtering, search, pagination) #632)
• Self-review completed
• Code follows project style guidelines

Testing

Run npx vitest run src/pages/Courses.test.tsx to verify all 12 tests pass.

Tests cover:

• Track filter: filtering by "Stellar Basics", "Advanced Topics", clearing
• Difficulty filter: filtering by difficulty level
• Search: by title and description (case-insensitive)
• Pagination: multiple pages, page navigation, filter reset
• Loading state: skeleton rendering
• Empty state: no results display
• URL params: reading filter state from searchParams
• Link hrefs: correct course slug URLs

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