fix(base): show skeleton while inline embed is being created, anchor at slash position

The Database slash command deleted the trigger text and then awaited
the create-base API before inserting the embed. During the wait the
editor sat empty (no visible state), and by the time the embed was
inserted the editor's selection had often drifted — so the new node
landed in the wrong place and the page appeared to "jump up" when the
response arrived.

Insert a placeholder baseEmbed node synchronously at the slash position
with pageId: null and a unique pendingKey. BaseEmbedView renders
BaseTableSkeleton while pendingKey is set — same skeleton BaseTable
shows on its own initial load, so the swap to the real table is a
visual no-op. Once the API responds, look up the placeholder by its
pendingKey and patch in the real pageId via setNodeMarkup. On API
failure, remove the placeholder and surface a toast. The pendingKey
attribute is non-serializing (parseHTML returns null, renderHTML
returns {}) so a placeholder can't survive a page reload as orphan.

packages/editor-ext/src/lib/base-embed/base-embed.ts

@@ -8,7 +8,10 @@ export interface BaseEmbedOptions {
 declare module '@tiptap/core' {
   interface Commands<ReturnType> {
     baseEmbed: {
-      insertBaseEmbed: (attrs: { pageId: string }) => ReturnType;
+      insertBaseEmbed: (attrs: {
+        pageId: string | null;
+        pendingKey?: string | null;
+      }) => ReturnType;
     };
   }
 }
@@ -32,6 +35,16 @@ export const BaseEmbed = Node.create<BaseEmbedOptions>({
         renderHTML: (attrs) =>
           attrs.pageId ? { 'data-page-id': attrs.pageId } : {},
       },
+      // Transient marker set when the slash command inserts the embed
+      // before the server has assigned a pageId. The view renders a
+      // skeleton in this state. Cleared once the API responds and the
+      // real pageId is patched in. Not serialized — embeds saved with
+      // a pendingKey would orphan if the page were closed mid-request.
+      pendingKey: {
+        default: null,
+        parseHTML: () => null,
+        renderHTML: () => ({}),
+      },
     };
   },