fix(workspace): extract replaces the branch in the editor, with undo

The server only creates the new note and returns the link; the editor
swaps the branch for it as an ordinary own edit through the sync engine.
No more clash with unsent typing (conflict copies), the link shows at
once also with polling, and Strg+Z restores the branch. Undoing (toast or
Strg+Z) puts the new note into the trash via POST /extract/undo. The
sidebar gets the new note right away and opens its folder.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -709,14 +709,19 @@ export const api = {
     return request<TrashedDocument[]>('/api/v1/documents/trash');
   },
 
-  /** Turns a bullet and its children into a note of its own; the bullet becomes a [[link]]. */
-  async extractBranch(docId: string, blockId: string, clientId: string): Promise<Document> {
-    return request<Document>(`/api/v1/documents/${docId}/extract`, {
+  /** Creates a note from a bullet's children; the editor then replaces the branch by `link` itself. */
+  async extractBranch(docId: string, blockId: string, clientId: string): Promise<{ document: Document; link: string }> {
+    return request<{ document: Document; link: string }>(`/api/v1/documents/${docId}/extract`, {
       method: 'POST',
       body: JSON.stringify({ blockId, clientId }),
     });
   },
 
+  /** Puts a note created by extractBranch into the trash again. */
+  async undoExtract(newDocId: string): Promise<void> {
+    await request(`/api/v1/documents/${newDocId}/extract/undo`, { method: 'POST' });
+  },
+
   /** Appends the source note to the target; the source goes to the trash. */
   async mergeDocuments(targetId: string, sourceId: string): Promise<MergeResult> {
     return request<MergeResult>(`/api/v1/documents/${targetId}/merge`, {