Replace hardcoded person share values with formulas in spreadsheet

[dphoria]

Updates the spreadsheet to use formulas instead of hardcoded values for person shares, making calculations transparent and editable.

Changes:

• Modified get_shares_spreadsheet() to generate formulas for each person's share of items
• Formula format: =$B{row}/{split_count} (e.g., =$B2/2 for items split between 2 people)
• Uses get_split_count() to determine how many people share each item
• Updated test expectations to verify formulas instead of literal values
• Makes spreadsheet calculations visible and manually editable

Benefits:

• Users can see exactly how shares are calculated
• Formulas can be edited if needed
• Split counts are clearly visible in the formulas