fix: guard EditorContent against null editor in tiptap 3.23.6

tiptap v3.19+ throws when EditorContent receives a null editor
(ueberdosis/tiptap#7529). useEditor() returns null on the first render
cycle before the instance is constructed — this was tolerated in 3.22.5
but crashes silently in 3.23.6, killing the entire ChannelPane subtree.

desktop/src/features/forum/ui/ForumComposer.tsx

@@ -451,7 +451,7 @@ export function ForumComposer({
             className="rich-text-composer max-h-32 overflow-y-auto"
             onKeyDown={handleEditorKeyDown}
           >
-            <EditorContent editor={richText.editor} />
+            {richText.editor && <EditorContent editor={richText.editor} />}
           </div>
 
           <MessageComposerToolbar

desktop/src/features/forum/ui/ForumComposerCompactLayout.tsx

@@ -31,7 +31,7 @@ export function ForumComposerCompactLayout({
         className="rich-text-composer max-h-10 min-w-0 flex-1 overflow-y-auto"
         onKeyDown={onEditorKeyDown}
       >
-        <EditorContent editor={editor} />
+        {editor && <EditorContent editor={editor} />}
       </div>
       <Button
         aria-label={isSending ? "Sending" : "Send message"}

desktop/src/features/messages/ui/MessageComposer.tsx

@@ -720,7 +720,7 @@ export function MessageComposer({
             ref={composerScrollRef}
             onKeyDown={handleEditorKeyDown}
           >
-            <EditorContent editor={richText.editor} />
+            {richText.editor && <EditorContent editor={richText.editor} />}
           </div>
 
           <MessageComposerToolbar