[WC-3334] Fix DG2 export type bugs

[samuelreichert]

Pull request type

Refactoring (e.g. file rename, variable rename, etc.)
Bug fix (non-breaking change which fixes an issue)

Description

Fix multiple data export bugs in DataGrid2's Excel export feature (WC-3334).

Problems

1. customContent columns ignore exportType — exportValue always exports as text regardless of exportType setting, making numeric values unusable for calculations in Excel
2. Date exports leak time components — Date-only formats (e.g., dd-MMM-yyyy) carry a hidden time component in the cell value, affecting sorting and calculations
3. Boolean exports show TRUE/FALSE — Excel boolean cells always render as TRUE/FALSE regardless of the w field; DataGrid2 displays Yes/No in the UI
4. Long numbers lose precision — Big.toNumber() loses precision for values >15 significant digits, causing scientific notation in Excel

Changes

• Extracted cell-creation logic from DSExportRequest.ts into standalone cell-readers.ts module (pure functions, zero behavior change)
• customContent reader now branches on exportType: parses to Number() for "number", new Date() for "date", with graceful string
  fallback on parse failure
• Added hasTimeComponent(format) / stripTime(date) helpers — when format has no h/s tokens, date values are normalized to midnight UTC
• excelBoolean now returns { t: "s", v: "Yes" | "No" } instead of { t: "b" } to match grid display
• Big values with >15 significant digits export as strings via toFixed() to preserve all digits
• 28 new unit tests covering all reader paths, edge cases, and fallback behavior

[iobuhov]

Can we use "proper" names? (e.g. type, value, format?

[samuelreichert]

Not if we want to keep it SheetJS compatible: https://docs.sheetjs.com/docs/csf/cell

[iobuhov]

Which format schema we use in number/date cases? Is there ISO spec or something?

[samuelreichert]

The z property follows ECMA-376 / OOXML number format strings, the same syntax Excel uses natively.
SheetJS reads them directly and applies them in the workbook.

Dates: yyyy, mm, dd, mmm, h, s tokens
e.g. "yyyy-mm-dd", "dd-mmm-yyyy hh:mm:ss"

Numbers: #, 0, ,, . tokens
e.g. "#,##0.00", "$0.00"

There's no ISO spec here; it's the Excel format code syntax defined in ECMA-376 Part 1, §18.8.31.

The hasTimeComponent regex /[hs]/i keys off h (hours) and s (seconds) from that same token set to decide whether to strip the time part before writing the date cell.