refactor(table): add grouped and pinned column headers to table component

[LWS49]

Summary

This PR adds support for multi-level grouped headers and pinned (sticky) columns to the table component. It introduces a new header building system that handles complex column hierarchies while maintaining proper column ordering and pinning behavior.

Key Changes

• New buildHeaderRows function (builder/buildHeaderRows.ts): Core logic that transforms a flat column array into a multi-row header structure, handling:

  • Multi-level column grouping via groupPath property
  • Column pinning to left/right edges with proper reordering
  • Automatic colSpan/rowSpan calculation for group cells
  • Development-time validation (missing widthPx on pinned columns, inconsistent grouping depths, non-contiguous groups)

• New computePinOffsets function (MuiTableAdapter/gridSxStyles.ts): Calculates sticky positioning offsets for pinned columns (cumulative from left, reverse cumulative from right)

• Updated MuiTableHeader component: Refactored to render multi-row headers with proper sticky positioning, pin attributes, and sort/filter UI only on leaf cells

• New useStickyHeaderOffsets hook (MuiTableAdapter/useStickyHeaderOffsets.ts): Dynamically computes top offsets for sticky header rows based on their heights

• Enhanced ColumnTemplate interface: Added groupPath and pin/widthPx properties to support grouped and pinned columns

• Updated MuiTableRow component: Added pinned column support with proper sticky positioning and offset calculation

• New grid styling (gridSxStyles.ts): Comprehensive styling for grouped headers, pinned columns, and their interactions (borders, z-index, sticky behavior)

• Comprehensive test coverage:

  • 30+ unit tests for buildHeaderRows covering flat/grouped/pinned scenarios and edge cases
  • 10+ integration tests for the full <Table /> component with various configurations

Notable Implementation Details

• Pinned columns are automatically reordered to appear first (left pins) and last (right pins) regardless of declaration order
• Pinned columns span all header rows when grouping is present
• Group cells only appear in non-leaf rows; leaf cells contain the actual column data
• Sticky header positioning accounts for multi-row headers by computing cumulative heights
• Development mode includes validation warnings for common configuration errors (non-contiguous groups, inconsistent depths)
• The maxHeight prop on TableTemplate enables scroll-contained tables with sticky headers