feat(workspace): voice notes via /audio (M3)

- /audio (only in an empty bullet) shows a recording bubble: green round
  button starts, it turns red with a live wave and timer on its left, a
  tap saves the recording as an inline audio player. An audio file can be
  uploaded instead; Esc/✕ restores the empty bullet.
- Chrome's WebM recordings get their length measured, so the player shows
  the total time and seeking works.
- With the own Whisper server "Transkribieren" turns the recording into
  the (searchable) caption.
- Loaded files keep their Blob, as the CSP rightly blocks fetch() on blob:
  URLs.

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

frontend/src/features/help/HelpView.tsx

@@ -160,6 +160,8 @@ export const HelpView: React.FC<HelpViewProps> = ({ onBack }) => {
           'callout',
           'divider',
           'bild',
+          'audio',
+          'sprachnotiz',
           'image',
           'foto',
           'auto-formatting',
@@ -199,6 +201,10 @@ export const HelpView: React.FC<HelpViewProps> = ({ onBack }) => {
                   <span className="text-accent font-bold">🖼️</span>
                   <span>/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>/audio (Sprachnotiz aufnehmen: grüner Knopf startet, roter speichert; oder Audiodatei hochladen)</span>
+                </div>
               </div>
             </div>
             <div className="pt-2 border-t border-border">