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/features/help/HelpView.tsx

@@ -160,6 +160,9 @@ export const HelpView: React.FC<HelpViewProps> = ({ onBack }) => {
           'callout',
           'divider',
           'bild',
+          'extract',
+          'auslagern',
+          'zusammenführen',
           'audio',
           'sprachnotiz',
           'diktat',
@@ -202,6 +205,10 @@ export const HelpView: React.FC<HelpViewProps> = ({ onBack }) => {
                   <span className="text-accent font-bold">🖼️</span>
                   <span>/image oder /bild (Bild hochladen, fotografieren, einfügen oder per Link; nur im leeren Punkt)</span>
                 </div>
+                <div className="p-1.5 bg-panel rounded border border-border flex items-center gap-1.5 text-main col-span-2">
+                  <span className="text-accent font-bold">📤</span>
+                  <span>/extract oder /notiz (Punkt mit allen Unterpunkten in eine eigene Notiz auslagern, an seiner Stelle bleibt ein [[Link]])</span>
+                </div>
                 <div className="p-1.5 bg-panel rounded border border-border flex items-center gap-1.5 text-main col-span-2">
                   <span className="text-accent font-bold">🎙️</span>
                   <span>/audio (Sprachnotiz aufnehmen: grüner Knopf startet, roter speichert; oder Audiodatei hochladen)</span>