feat(grouped-table): data-table style parity + slot className overrides

[yahyafakhroji]

Summary

Follow-up to the GroupedTable release — aligns GroupedTable's styling with data-table and adds slot-level className overrides.

• Style parity: header/body/cells use the shared base table primitives like data-table; cell truncate removed (long content shows via the horizontal scroll, same as data-table).
• Group band → section header: the collapsible band now reads as a muted sub-header (h-10, px-2, font-medium, text-muted-foreground, border-b) with a subtle bg-muted/40 fill so groups stay distinct.
• Slot className API (mirrors data-table ContentProps): tableClassName, headerRowClassName, headerCellClassName, groupHeaderClassName (string | (group) => string), bodyClassName, rowClassName (string | (row) => string), cellClassName (string | (cell) => string), toolbarClassName. All optional, merged on top of defaults via cn().

Test plan

• 41 unit tests pass (incl. new: string cellClassName on cells; groupHeaderClassName(group) on band)
• typecheck clean · lint 0 errors · build emits dist
• Visual pass in Storybook (light + dark): band-as-header + no-truncate confirmed
• Optional reviewer eyeball in Storybook

Changeset: minor.