fix(alarm): silence the fire channel explicitly instead of by omission

Omitting setSound on a NotificationChannel leaves the platform DEFAULT
notification sound active — omission is not silence. The v3 channel
now calls setSound(null, null) exactly like the pre-notice channel
does, so the native STREAM_ALARM player stays the ring's only audible
source. Caught by verification against design D4 before any build.

android/app/src/main/kotlin/es/freetimelab/pluriwave/PluriWaveAlarmService.kt

@@ -674,17 +674,21 @@ class PluriWaveAlarmService : Service() {
             migrateLegacyChannels(context, manager)
             // Re-create each time (not early-returning when present) so the
             // localized name/description refresh after a locale change. Android
-            // updates name + description on an existing channel; importance
+            // updates name + description on an existing channel; importance and
-            // stays fixed from first creation. No setSound call: this channel
+            // sound stay fixed from first creation. setSound(null, null) is
-            // is silent by construction (Requirement: Fire notification posts
+            // REQUIRED for silence: omitting the call leaves the platform
-            // with no sound) -- the native MediaPlayer on STREAM_ALARM is the
+            // DEFAULT notification sound on the channel (same reason the
-            // only audible source, so a channel sound would double it.
+            // pre-notice channel calls it explicitly). This channel must be
+            // silent (Requirement: Fire notification posts with no sound) --
+            // the native MediaPlayer on STREAM_ALARM is the only audible
+            // source, so a channel sound would double it.
             val channel = NotificationChannel(
                 CHANNEL_ID,
                 AlarmNotificationStrings.fireChannelName(context),
                 NotificationManager.IMPORTANCE_HIGH
             ).apply {
                 description = AlarmNotificationStrings.fireChannelDescription(context)
+                setSound(null, null)
                 enableVibration(true)
             }
             manager.createNotificationChannel(channel)