Fix cheat sheet editor and saved-sheet state issues

[bdtran2002]

Summary

• remove the invalid PDF prewarm request on page load and make the middle compile action always compile the exact LaTeX currently in the left editor
• add syntax-highlighted editor rendering plus compile-error line highlighting so failed LaTeX compiles stay visible and actionable in place
• fix clear/reset persistence so clearing a sheet also clears saved sheet identity, undo history, and stale formula storage across reloads
• keep formula checkbox state in sync after manual removals from the ordered list instead of dropping or restoring the wrong selections
• make saved-sheet full-document LaTeX passthrough inject practice problems correctly and use extarticle for wrapped 8pt/9pt documents

Validation

• npx eslint src
• npm run build
• python3 -m py_compile backend/api/models.py backend/api/tests.py
• "/tmp/cheat-sheet-verify-venv/bin/python" backend/manage.py test api.tests
• live browser smoke test against local frontend/backend:
  • select CALCULUS I and generate a sheet
  • insert \\badcommand into the editor and confirm the error banner/highlighted line appear while the edited content stays intact
  • restore valid LaTeX and recompile successfully
  • clear the sheet, reload, and confirm currentCheatSheet is blank, cheatSheetData is removed, and no formulas remain selected

Notes

• updated existing PR Fix cheat sheet editor and saved-sheet state issues #50 for branch bt/save-and-config-fixes
• AGENTS.md was not changed, committed, or pushed