refactor(#51): extract mastery threshold constants to shared file

[atbrace]

Summary

• Created src/lib/constants.ts with MASTERY_THRESHOLD (85), APPROACHING_THRESHOLD (60), READINESS_APPROACHING_THRESHOLD (65), EXAM_PREP_THRESHOLD (75), and MASTERY_THRESHOLD_DECIMAL (0.85)
• Updated 10 callsite files to import from constants instead of using magic numbers
• Zero magic threshold numbers remain in source files (only in test description strings)

Closes #51

Test plan

• pnpm test:run — 437 tests pass
• grep -rn ">= 85" src/ --include="*.ts" --include="*.tsx" returns only test descriptions
• No behavioral changes — pure mechanical refactor

🤖 Generated with Claude Code