feat: add DSA mode with Queue data structure exercises

- Implement DSA command for data structures learning mode
- Add 6 progressive Queue exercises (FIFO implementation)
- Add category filter to ExerciseManager for DSA-only navigation
- Add reset functionality (x key) for DSA exercises
- Fix queue3.swift missing main() call
- Update README with DSA mode documentation

Author: tornikegomareli

README.md

@@ -97,6 +97,18 @@ swiftlings reset variables1
 
 Restores an exercise to its original state.
 
+### Data Structures & Algorithms Mode (NEW!)
+
+Start the DSA learning mode to master data structures through implementation:
+
+```bash
+swiftlings dsa
+```
+
+Currently includes Queue exercises (6 progressive exercises building a FIFO data structure from scratch). New data structures and algorithms are coming soon! 
+
+I'm open to PRs for improving existing DSA exercises or adding new data structures and algorithms.
+
 ## Exercise Structure
 
 Exercises are organized into 19 progressive categories: