feat: column auto-fit (double-click divider + context menu)

[datlechin]

Summary

Add native macOS column auto-fit functionality to DataGridView, matching Finder/Numbers behavior.

Features

• Double-click column divider → auto-fit that column to content (sizeToFitWidthOfColumn: delegate)
• Right-click header → "Size to Fit" → auto-fit the clicked column
• Right-click header → "Size All Columns to Fit" → auto-fit every column

Implementation

• New calculateFitToContentWidth() in DataGridCellFactory — no max-width (800px) or max-chars (50) caps, unlike the initial-load calculation
• All three features reuse the same uncapped width calculation
• Works in both table data tab and SQL query results tab (shared DataGridView)
• Column widths persist after auto-fit via existing ColumnLayoutStorage

Test plan

• Double-click a column divider in header → column resizes to fit content
• Right-click column header → "Size to Fit" → column resizes
• Right-click column header → "Size All Columns to Fit" → all columns resize
• Verify no max-width cap — wide content columns expand fully
• Verify works in table tab (browsing data)
• Verify works in query results tab (after running SQL)
• Verify resized widths persist when switching tabs