Prepend UTF-8 BOM to generated CSV files

[joshuali925-osdbot]

Description

Prepends a UTF-8 BOM (\uFEFF) to generated CSV files so that Excel and other spreadsheet applications correctly detect the encoding. This fixes the issue where special characters (e.g. ') display as garbled text (’) when opening CSV exports in Excel.

Changes

• Added UTF-8 BOM prefix in convertToCSV() in dataReportHelpers.ts
• Added unit tests verifying BOM presence and correct CSV content

Testing

Unit tests pass:

PASS server/routes/utils/__tests__/dataReportHelpers.test.ts
  convertToCSV
    ✓ should prepend UTF-8 BOM to CSV output
    ✓ should contain correct CSV content after BOM

Issues Resolved

Resolves #545

[joshuali925]

@joshuali925-osdbot can you generate and upload reporting csv files with and without the BOM commit

[joshuali925-osdbot]

Here are sample CSV files generated with special characters (curly quotes, accented letters, CJK, symbols):

• Without BOM: 749-without-bom.csv
• With BOM: 749-with-bom.csv

The with-BOM file starts with EF BB BF (UTF-8 BOM), which tells Excel to use UTF-8 encoding. Without it, Excel defaults to the system locale encoding and garbles special characters.

Sample data used:

name,city,notes
John's Report,São Paulo,€100 — paid
"Jane ""Doe""",Zürich,résumé – Pro™
日本語テスト,北京,中文数据 «test»

To test: open each file in Excel — the without-BOM file will show garbled characters like ’ instead of ', while the with-BOM file displays correctly.