perf: skip column width calculation when saved layout exists

[datlechin]

Summary

• calculateOptimalColumnWidth() samples 30 rows per column to estimate width. During column rebuild, this ran for EVERY column even when the user had saved column widths that would immediately overwrite the calculated values.
• Now skips the calculation when columnLayout.columnWidths is non-empty, using saved widths directly. For a 50-column table, this eliminates 50 x 30 = 1,500 cell reads on every column rebuild.
• Applies to both full rebuild (columns changed) and in-place update (initial data load) paths.

Test plan

• Open a table for the first time (no saved layout) — columns auto-size correctly
• Resize a column, switch tabs and back — verify saved width is preserved (no flash of auto-sized width)
• Open a table with saved layout — should load noticeably faster for wide tables (50+ columns)
• Delete saved layout (clear column widths) — verify auto-sizing kicks back in