Embed: support vertical category layout to match group view

## Summary

The embed configurator at `/embed-setup` allows selecting multiple categories and subcategories, which is helpful for landscapes where a group spans multiple top-level categories. However, the embed renders categories in a **horizontal** orientation, which differs from the full landscape group view (e.g. `?group=organizations`) where categories are displayed **vertically** in the left-hand sidebar.

## Current behavior

When embedding with multiple categories selected, the category bar is rendered horizontally above the subcategory list. This layout works for a single category but becomes cramped and visually inconsistent when four or more categories are selected together.

## Desired behavior

An option in the embed configurator to render the category list **vertically** (left-side column), matching the layout used in the live group view. This would allow an embedded multi-category view to look and feel consistent with the full landscape site.

## Context

This came up while working on the [CAMARA Ecosystem Landscape](https://camara.landscape2.io), which has two groups: Members (single top-level category, embeds cleanly) and Organizations (four top-level categories: Planning, Integration, Solution Provider, and Operation). The multi-category embed workaround covers the data need, but the horizontal layout is a UX gap compared to the native group view.

Confirmed as a current gap by @cynthia-sg in the landscape2 Slack community (2026-04-08).

## References

- Live example of the desired vertical layout: https://camara.landscape2.io/?group=organizations
- Embed setup page: https://camara.landscape2.io/embed-setup