Add setting to skip header row in calculations

[Cattamer]

Summary

• Adds a new "Skip header row" toggle in settings
• When enabled, the first row of each table is excluded from vertical (^) calculations
• Useful when header rows contain numbers that shouldn't be included in sums, averages, etc.

Problem

When table headers contain numbers (e.g., "Q1 2024", "Item #", or numeric column labels), these get included in vertical calculations like sum^ or avg^, producing incorrect results.

Solution

A simple toggle that, when enabled, starts vertical calculations from row index 1 instead of 0, effectively skipping the header row.

Test plan

• Enable "Skip header row" in settings
• Create a table with numbers in the header row
• Verify that sum^, avg^, etc. exclude the header values
• Disable the setting and confirm headers are included again

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