data_table: #5 Add independently resizable columns + fix variable list scrolling

[HalavicH]

This PR adds spreadsheet-style independently resizable columns (dragging changes total table width) and fixes scrolling issues in variable list mode.

What changed:

• Adds ResizableColumnsState struct for independently resizable columns (spreadsheet-style)
• Adds ColumnWidthConfig::Resizable variant for spreadsheet mode
• Adds DraggedResizableColumn drag payload type
• Adds ResizableHeaderInfo for double-click-to-reset functionality
• Adds render_resize_handles_resizable function for resize handles rendering
• Adds horizontal scroll handle to TableInteractionState
• Adds .on_drag_move::<DraggedResizableColumn> handler to Table for drag resizing

Bug fixes:

• Fixed missing vertical scrollbar in variable list mode
• Fixed half-broken scrolling in variable list mode (added .measure_all() to make scrollbar aware of the height of the table)
• Moved vertical scrollbar to be pinned at the right side of the pane with table — previously it was attached to the table content and was pushed off-screen when table content was too wide

API addition:

// New variant added:
pub enum ColumnWidthConfig {
    Static { widths: StaticColumnWidths, table_width: Option<DefiniteLength> },
    Redistributable { entity: Entity<RedistributableColumnsState>, table_width: Option<DefiniteLength> },
    Resizable(Entity<ResizableColumnsState>),  // NEW: spreadsheet-style
}

Callers updated:

• csv_preview: Changed from ColumnWidthConfig::redistributable() to use new resizable mode
• git_graph: Added resizable_info parameter

Context:

This is part 3 of a 3-PR series improving data table column width handling:

1. #51059 - Extract modules into separate files (mechanical change)
2. #51120 - Introduce width config enum for redistributable column widths (API rework)
3. This PR: Add independently resizable columns + fix variable list scrolling (new feature + bug fixes)

The series builds on previously merged infrastructure:

• #46341 - Data table dynamic column support
• #46190 - Variable row height mode for data tables

Primary beneficiary: CSV preview feature (#48207)

This work is based on the original draft PR #44344, decomposed into reviewable pieces.

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Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

• N/A or Added/Fixed/Improved ...