feat(nests): empty-stage hint, local hush, moderator/force-mute moderation

Three improvements stacked into one commit since they share files:

#3 Empty-stage hint: StageGrid no longer disappears when nobody is on
   stage. The "Stage" label stays and a quiet "Waiting for speakers…"
   line keeps the strip visible so the room doesn't look broken before
   the first speaker arrives.

#5 Local per-speaker hush: AudioPlayer gains a setVolume(Float)
   default-no-op method; AudioTrackPlayer composes mute and volume
   multiplicatively into AudioTrack.setVolume so a hushed stream stays
   silent regardless of mute state. NestViewModel exposes
   locallyHushed: ImmutableSet<String> in NestUiState plus
   setLocalHushed(pubkey, hushed) — applied at attach time so a
   re-subscribe of an already-hushed speaker stays silent. New "Hush
   this speaker" / "Restore this speaker" row in
   ParticipantHostActionsSheet, available to anyone (it affects only
   our own playback, nothing on the wire).

#4 Host moderation gaps: wires Promote-to-Moderator using the existing
   RoomParticipantActions.setRole(ROLE.MODERATOR) builder — UI gap
   only, no protocol change. Adds a new AdminCommandEvent.Action.MUTE
   variant + AdminCommandEvent.forceMute(room, target) builder; the
   sheet emits it on "Force-mute speaker" and the
   AdminCommandsCollector dispatches incoming MUTE actions to a new
   NestViewModel.onForceMuted() that routes through the existing
   setMicMuted(true) path. Honor-based, same trust model as KICK —
   relays don't enforce signer authority, the client checks the
   signer is host or moderator on the active kind-30312.

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/experimental/nests/admin/AdminCommandEvent.kt

@@ -71,6 +71,16 @@ class AdminCommandEvent(
         val code: String,
     ) {
         KICK("kick"),
+
+        /**
+         * Force the target speaker's mic to muted. Honor-based: the
+         * target's client receives the command and flips its own
+         * presence-side mic-mute. A non-cooperating client could
+         * ignore the event and keep broadcasting — same trust model
+         * as KICK, which a misbehaving client could also ignore by
+         * not disconnecting.
+         */
+        MUTE("mute"),
         ;
 
         companion object {
@@ -95,5 +105,18 @@ class AdminCommandEvent(
             aTag(room)
             add(PTag(target).toTagArray())
         }
+
+        /**
+         * Build a force-mute command. Same envelope as [kick] —
+         * tagged for [room] + [target], action verb in content.
+         */
+        fun forceMute(
+            room: ATag,
+            target: HexKey,
+            createdAt: Long = TimeUtils.now(),
+        ) = eventTemplate<AdminCommandEvent>(KIND, Action.MUTE.code, createdAt) {
+            aTag(room)
+            add(PTag(target).toTagArray())
+        }
     }
 }