Refactor column variants: `panel` & `compact`

[dawoodkhan82]

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Is your feature request related to a problem? Please describe.
For Column variants:
panel: outer border, rounded container, panel background, and padding. Children retain their normal rounded corners and automatic Form grouping.
compact: the same outer panel styling, but additionally removes child/.box border radii and disables automatic Form wrapping: BaseColumn.svelte (line 78), column.py (line 66).

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