feat(workspace): turn a branch into its own note, merge notes

- Extract: button on the bullet (desktop) or /extract (/notiz). The bullet
  becomes the title (hashtags stay at the link), its children the content
  of a new note in the same folder with the source's shares (a foreign
  source owner gets write access); the branch becomes [[title]] through
  the operation log, so open editors follow.
- Merge: drop a note onto another in the sidebar or use "Mit anderer
  Notiz zusammenführen…" in the note header. The own source is appended
  as an h2 section with its bullets below and goes to the trash; the toast
  offers a complete undo.

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

frontend/src/lib/api.ts

@@ -36,6 +36,7 @@ import {
   User,
   UserInvite,
   VoiceAdminStatus,
+  MergeResult,
   WSMessage,
 } from './types';
 import { extractHashtags } from './hashtags';
@@ -708,6 +709,29 @@ 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/${docId}/extract`, {
+      method: 'POST',
+      body: JSON.stringify({ blockId, clientId }),
+    });
+  },
+
+  /** Appends the source note to the target; the source goes to the trash. */
+  async mergeDocuments(targetId: string, sourceId: string): Promise<MergeResult> {
+    return request<MergeResult>(`/api/v1/documents/${targetId}/merge`, {
+      method: 'POST',
+      body: JSON.stringify({ sourceId }),
+    });
+  },
+
+  async undoMerge(m: MergeResult): Promise<void> {
+    await request(`/api/v1/documents/${m.targetId}/merge/undo`, {
+      method: 'POST',
+      body: JSON.stringify({ sourceId: m.sourceId, headingId: m.headingId }),
+    });
+  },
+
   async restoreDocument(id: string): Promise<void> {
     await request(`/api/v1/documents/${id}/restore`, { method: 'POST' });
   },