Add toggle for Markdown preview in editor mode

[perber]

When editing a page, it should be possible to quickly toggle the Markdown preview on and off directly in the editor mode. This makes it easier to switch between a focused editing view and a live preview without changing context. We should store the setting for the user in the browser state.

Acceptance Criteria

• A visible control (e.g. button or icon) in the editor allows toggling the preview.
• Editor-only mode: only the CodeMirror editor is visible.
• Preview mode: editor and rendered Markdown preview are visible (or an easy switch between them).
• The toggle state is applied without reloading the page or losing editor content.
• (Optional) A keyboard shortcut exists to toggle the preview while editing.

[perber]

• (Optional) A keyboard shortcut exists to toggle the preview while editing.

Keyboard handling will be a separate topic.