Color Picker presets gap and DataGrid expand column HeaderClass (#598, #597)

[mathewtaylor]

Two small fixes from open issues.

Color Picker: gap under "Presets" (#598)

Closes #598

• The "Presets" label is now bb:block. space-y-2 (Tailwind v4) puts a bottom margin on each child except the last, and an inline span ignores it, so the gap above the swatches never showed.
• Removed bb:space-y-2 from the Hue and Alpha slider wrappers. Each has one child, so the class did nothing.

No API change.

DataGrid: HeaderClass on BbDataGridExpandColumn (#597)

Closes #597

• New HeaderClass parameter, passed to the grid like on the Select, Property and Template columns. The grid already merges it last, so it overrides the default width and padding.
• Demo: the "Colored Header" example on the DataGrid styling page now includes an expand column. Added an API Reference entry.
• API surface snapshot updated (one added line).
• Tests: ExpandColumnTests checks the value reaches the grid.

Tests run

• ExtraAttributeTests, CultureInvariantStyleTests (Color Picker): 22 passed
• ExpandColumnTests, ComponentsApiSurfaceTests: 3 passed
• BlazorBlueprint.Demo.Shared builds

Neither change has been checked in a browser.

[mathewtaylor]

Closing as a duplicate: #597 shipped in #602, and #598 is in #603.