fix(google-sheets): normalise header case and fix monthly COUNTIFS formula

[sgresh-stripe]

Summary

Fixes PR comments from here: #369

• Header case normalisation: fieldToDisplay now lowercases each word segment before capitalising the first, preventing mixed-case field names (e.g. API_version) from leaking uppercase into column headers.
• Monthly count formula: Replaces COUNTIFS with SUMPRODUCT + DATEVALUE(LEFT(...,10)) to correctly parse ISO timestamp strings into Sheets date serials; the previous COUNTIFS approach failed on text-formatted dates.
• Tests: Adds a fieldToDisplay / displayToField roundtrip test suite covering snake_case, system fields, and case normalisation.

Test plan

• pnpm test passes (unit tests cover new fieldToDisplay/displayToField cases)
• Manually verify Google Sheets header row shows correct sentence-case labels for mixed-case source fields
• Manually verify "New Customers by Month" example section counts correctly in a real spreadsheet