feat: add freeze columns with multi-select and opaque sticky backgrounds

[VauntlekV]

What

Adds freeze-column support in the webview:

• Freeze/Unfreeze/Clear Frozen COLUMNS context-menu actions
• Persist frozenColumns in webview state and reapply on chunk loads/updates/width changes
• Support shift multi-select to freeze selected header ranges
• Use stable opaque sticky backgrounds to prevent selection highlight bleed-through
• Add webview freeze-column source assertions

Why

Freezing columns improves readability when horizontally scrolling wide CSVs, especially when comparing fields across many columns.

How to test

• Open a wide CSV in the webview
• Right-click a column header → Freeze Columns; scroll horizontally and confirm frozen columns stay fixed
• Shift-select multiple column headers → freeze; confirm the selected range is frozen
• Reload / trigger chunk load / resize columns; confirm frozen columns persist and reapply correctly
• Verify selection highlight does not bleed through frozen cells

Notes

No breaking changes intended.

[jonaraphael]

Thank you for the freeze-column work. The release now includes persistent leading frozen rows and columns through #117. This PR also supports choosing other column ranges, which is separate behavior, so I am leaving it open rather than merging that additional scope without review. It would need rebasing onto the new frozen-pane implementation and focused tests for selection, reordering, chunks and theme interactions.