Flowchart add term + undo/redo history tracker

[alexhtruong]

📌 Summary

Users can now add summer terms to the flowchart and undo/redo actions on the flowchart(non-persisting)

🛠 Changes Made

• Reorganized and updated some flowchart-related files
• Summer term add button
• Undo/redo history tracker(saves flowchart state on each action and adds to undo/redo stack

✅ Checklist

• My code follows the PolyLink Contribution Guidelines.
• I have tested my changes to ensure they work as expected.
• I have documented my changes (if applicable).
• My PR has a clear title and description.

📸 Screenshots (if applicable)

N/A

❓ Additional Notes

Should work for mobile - tested on browser