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/components/editor/autocomplete/UserMentionAutocomplete.tsx

@@ -41,6 +41,7 @@ function UnknownMentionItem({
     <MenuItem
       as="button"
       radii="300"
+      onMouseDown={(e: React.MouseEvent) => e.preventDefault()}
       onKeyDown={(evt: ReactKeyboardEvent<HTMLButtonElement>) =>
         onTabPress(evt, () => handleAutocomplete(userId, name))
       }
@@ -162,6 +163,7 @@ export function UserMentionAutocomplete({
               key={roomMember.userId}
               as="button"
               radii="300"
+              onMouseDown={(e: React.MouseEvent) => e.preventDefault()}
               onKeyDown={(evt: ReactKeyboardEvent<HTMLButtonElement>) =>
                 onTabPress(evt, () => handleAutocomplete(roomMember.userId, getName(roomMember)))
               }