Add the complete Solana Security Auditor learning path:
- Arithmetic and Accounting Exploits (difficulty 2)
- Protocol-Level Attacks (difficulty 3)
- Auditing Solana Programs (difficulty 3)
- Formal Verification with Kani (difficulty 3)

Add cross-course link-backs so the 5 courses read as a cohesive
path rather than standalone units. Each course introduction now
references preceding courses, and shared concepts (share inflation,
fee arithmetic, oracle manipulation) are explicitly cross-referenced
where they recur.

Fix hardcoded "Beginner" badge in CourseCard component — the difficulty
prop was computed but never used, so all courses displayed as Beginner
regardless of their actual difficulty level.