fix: resolve column corruption in CSV downloads.

[Sweety3106]

Related Issue

None (Self-discovered formatting bug)

Summary

This PR fixes a bug in the CSV download export where columns would shift and corrupt the spreadsheet if tasks contained commas (,), double quotes ("), or newlines (\n) in their text fields. We now use RFC-4180 compliant escaping on all fields before joining them.

Changes Made

• Added an escapeCSVField helper function to handle quote-escaping and wrap fields with commas or quotes in double-quotes.
• Updated the row mapper in backend/controllers/csvDownload.controller.js to apply escapeCSVField to all exported columns.

Testing

• Created a task with commas and quotes in the title (CS Homework, Part "1") and notes (Read pages 1, 2, and 3) and exported to CSV.
• Verified that the fields are properly escaped ("CS Homework, Part ""1""" and "Read pages 1, 2, and 3") and alignment is preserved.

Screenshots

None (Backend change only)

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)