fix: prevent editor blur on mouse click in autocomplete menus

Add onMouseDown preventDefault to all autocomplete suggestion MenuItems
so clicking a suggestion keeps the editor focused. Without this, the
mousedown event blurs the editor before onClick fires, causing Slate's
ReactEditor.toDOMNode to fail with "Cannot resolve a DOM node from Slate
node: {"text":""}" when Transforms.collapse tries to sync the selection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/room/CommandAutocomplete.tsx

@@ -85,6 +85,7 @@ export function CommandAutocomplete({
           as="button"
           radii="300"
           style={{ height: 'unset' }}
+          onMouseDown={(e: React.MouseEvent) => e.preventDefault()}
           onKeyDown={(evt: ReactKeyboardEvent<HTMLButtonElement>) =>
             onTabPress(evt, () => handleAutocomplete(commandName))
           }