List: child items not indented in rokkit theme; missing gap before group headers

[jerrythomas]

Description

Two related visual issues in the List component with the rokkit theme:

1. No indentation on child items — group headers and their children appear at the same visual indent level. The data-level attribute is set correctly, but the rokkit theme CSS does not apply indentation based on it. Child items should be inset relative to their group header.

2. No extra spacing before group headers — there is no visual gap between the last child item of a group and the next group header. Each group header should have extra top-spacing to separate it from the preceding items.

Proposed Fix

• Add an indent boolean prop to <List> (default true) that controls whether children are indented relative to their group header. Theme CSS should apply padding-left based on data-level when indent is active.
• Add appropriate margin-top or padding-top to [data-list-group] elements in the rokkit theme CSS.

Affected Files

• packages/ui/src/components/List.svelte — add indent prop
• packages/themes/src/rokkit/ — fix group spacing and child indentation CSS