✨(frontend) add "mute all participants" feature for admins

Implement admin capability to mute everyone's microphone in large rooms
where participants forget to mute themselves and are hard to identify
quickly.

Feature requested by @arnaud-robin. Provides instant room-wide muting
without individual confirmation popups, enabling efficient moderation
in busy conference scenarios.

src/frontend/src/locales/nl/rooms.json

@@ -443,6 +443,7 @@
     "raisedHands": "Opgestoken handen",
     "lowerParticipantHand": "Laat {{name}}'s hand zakken",
     "lowerParticipantsHand": "Laat alle handen zakken",
+    "muteParticipants": "Alle microfoons dempen",
     "waiting": {
       "title": "Wachtkamer",
       "accept": {