Add syntax selection for instructor-authored exercises

[sidprasad]

Summary

• add per-exercise syntax setting for instructor-authored exercises (Classic/Forge/Electrum)
• add student start-time syntax chooser for instructor exercises without a fixed syntax
• render instructor exercise formulas in the resolved syntax before display and show active syntax in the exercise view
• persist ltlsyntax cookie from the chosen/resolved syntax to keep UI consistent

Validation

• python -m py_compile src/app.py src/authroutes.py
• pytest -q test/test_parsing.py (fails in this environment because inflect is missing)

Notes

• left unrelated change in experiments/preprocess_responses.ipynb untouched