Opening an .xlsx turns 0 and negative date/time cells into text ("1899-12-31"), and export writes the text back #241

Found while reviewing PR #239 (#219) on a production build of b4566f9. The bug is on main before #239 too: src/lib/excelIO.ts is not touched by that PR.

What happens

A real .xlsx (written with ExcelJS) holding numbers under date/time formats, opened with Open file:

Because the values became text:

• =A2+2 gives #VALUE! (Excel: 1).
• SUM skips them in silence. A timesheet with a negative balance in [h]:mm adds up wrong.
• Export writes the text back. The exported file holds the strings "1899-12-31", "1899-12-30" and "1899-12-29 12:00" where the numbers were. The data is lost on a round trip.
• Same at 390px. No page error, no alert, and the "what this file cannot keep" report (บอกผู้ใช้ตอนเปิดไฟล์ ว่าไฟล์นี้มีอะไรที่แอปเก็บไว้ไม่ได้ · Tell the user, on open, what in this file will not survive a save #83) says nothing.

Values typed or computed in the app are fine after #239: a formula giving −1 under dd/mm/yyyy shows ########, and TEXT(0,"dd/mm/yyyy") is 00/01/1900. Only the import path is wrong.

Cause

ExcelJS hands a date-formatted cell over as a JS Date. rawForDate in src/lib/excelIO.ts turns that into ISO text with isoFromSerial. A serial of 0 or below becomes a year-1899 string, which dateLiteral/dateOf refuse (year < 1900), so the cell keeps the string as text. A midnight 0 under a time-only format already comes through as 00:00, and that case is fine.

Acceptance

• A cell of 0 or a negative number under a date or time format opens as that number, with its format kept. It then shows what Show zero and negative date serials as Excel does (#219) #239 shows: 00/01/1900 for 0 and ######## for a negative.
• Formulas reading those cells get the numbers: =A2+2 is 1, and the SUM above is −1.
• Export writes the numbers back with their formats. Check by reading the exported file with ExcelJS.
• A test that fails on today's main for each of: import, a formula reading the cell, and export.
• Positive dates and times open exactly as they do today. Two to check are 45366 → 15/03/2024 and 1.5 → 36:00.

Test file used: one sheet, the values and formats in the table above, plus =A5-A5-1 and =SUM(A2:A4).