Add --spreadsheet option to show column letters in the header

[ChrisJr404]

This adds an opt-in --spreadsheet flag that displays spreadsheet-style column letters (A, B, C, ..., Z, AA, ...) alongside the column names in the header, addressing #87.

The motivation from that issue is being able to refer to columns by letter the way you would in Excel or Google Sheets ("look at column Z"). When --spreadsheet is passed, each header cell is prefixed with its letter, e.g. [A] country, [B] population.

A few implementation notes:

• The letters are derived from each column's original position in the file (origin_index), not its on-screen position, so they stay correct and stable when scrolling horizontally or when a columns filter is active.
• The letter prefix is produced in the existing get_effective_column_name path, the same place the sort indicator is added, so it composes with sorting and flows through column-width computation with no changes to the layout/border rendering.
• The option is off by default; nothing changes unless --spreadsheet is passed.

I went with prefixing the existing header row (rather than adding a separate letters row) to keep the change small and avoid touching the border/offset math, but I'm happy to adjust the presentation if you'd prefer something different.

Testing: added a rendered-buffer snapshot test (test_spreadsheet_column_letters) and a unit test for the letter conversion covering the A/Z/AA/ZZ/AAA boundaries. cargo test --features cli passes (108 tests), and cargo fmt --check and cargo clippy --features cli --lib are clean.