feat: add cheatsheet page for all 21 languages

- Add /[language]/cheatsheet route with interview-focused method references
- Include 40-50 curated entries per language with syntax, examples, time/space complexity
- Desktop table view and mobile card view with responsive design
- Category and priority filters, search, and print functionality
- Quick navigation sidebar for category jumping
- Update layout to include Cheatsheet in mode navigation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: davidagustin

app/[language]/cheatsheet/page.tsx

@@ -16,6 +16,7 @@ const MODES = [
   { slug: 'quiz', label: 'Quiz', icon: '🧠' },
   { slug: 'exercises', label: 'Exercises', icon: '🔄' },
   { slug: 'reference', label: 'Reference', icon: '📚' },
+  { slug: 'cheatsheet', label: 'Cheatsheet', icon: '📋' },
   { slug: 'interview', label: 'Interview', icon: '🤖' },
 ] as const;