feat: implement unified step positioning for tutorial editor

Implements a unified step positioning system that allows concept steps
and practice steps to be inserted at correct logical positions in the
tutorial flow based on which "New" dropdown button is clicked.

Key changes:
- Add position property to TutorialStep interface for unified ordering
- Update addStep and addPracticeStep functions to calculate proper positions
- Use fractional positioning to insert between existing steps
- Ensure both step types participate in a single ordered sequence

🤖 Generated with [Claude Code](https://claude.ai/code)

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

apps/web/src/types/tutorial.ts

@@ -23,6 +23,7 @@ export interface TutorialStep {
     hint: string
   }
   multiStepInstructions?: string[]
+  position?: number // Position in unified tutorial flow
 }
 
 // Skill-based system for practice problem generation