fix(rte): guard editor.value in nextTick with optional chaining

The synchronous null check doesn't protect against the component
unmounting between the watcher firing and the nextTick callback
running — useEditor's onUnmounted sets editor.value to null, which
would cause a throw. Optional chaining handles this safely.

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

Author: akolson

contentcuration/contentcuration/frontend/shared/views/TipTapEditor/TipTapEditor/TipTapEditor.vue

@@ -207,7 +207,7 @@
           }
           if (newMode === 'edit' && editor.value && props.autofocus) {
             nextTick(() => {
-              editor.value.commands.focus('end');
+              editor.value?.commands.focus('end');
             });
           }
         },