feat(notifications): add branded monochrome icon and color to all notifications

Replace generic system icons (info bubble, stock alarm clock) with a
custom equalizer-bars vector drawable across all 4 notification
builders: pre-notice, snooze countdown, ringing alarm, and the audio
player. Apply the app's cyan brand color to the 3 alarm notifications
that previously had none. Audio notification now explicitly declares
its icon instead of falling back to the full-color launcher icon,
which Android was auto-silhouetting into an illegible status-bar
blob.

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

@@ -577,7 +577,8 @@ class AlarmScheduler(private val context: Context) {
         )
 
         val notification = NotificationCompat.Builder(appContext, PluriWaveAlarmReceiver.CHANNEL_ID)
-            .setSmallIcon(android.R.drawable.ic_lock_idle_alarm)
+            .setSmallIcon(R.drawable.ic_stat_pluriwave)
+            .setColor(NotificationBrand.CYAN)
             .setContentTitle(spec.title)
             .setContentText(text)
             .setPriority(NotificationCompat.PRIORITY_LOW)

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

@@ -0,0 +1,14 @@
+package es.freetimelab.pluriwave
+
+import androidx.annotation.ColorInt
+
+/**
+ * Shared brand color for native notification icons.
+ *
+ * Single source of truth for the cyan tint applied via `NotificationCompat.Builder.setColor()`
+ * across all PluriWave alarm and audio notifications, mirroring the [AlarmNotificationStrings]
+ * shared-constants precedent.
+ */
+object NotificationBrand {
+    @ColorInt const val CYAN: Int = 0xFF21D4D9.toInt()
+}

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

@@ -181,7 +181,8 @@ class PluriWaveAlarmReceiver : BroadcastReceiver() {
         )
 
         val notification = NotificationCompat.Builder(context, CHANNEL_ID)
-            .setSmallIcon(android.R.drawable.ic_dialog_info)
+            .setSmallIcon(R.drawable.ic_stat_pluriwave)
+            .setColor(NotificationBrand.CYAN)
             .setContentTitle(title)
             .setContentText(contentText)
             .setPriority(NotificationCompat.PRIORITY_LOW)

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

@@ -387,7 +387,8 @@ class PluriWaveAlarmService : Service() {
         snoozeMinutes: Int
     ) =
         NotificationCompat.Builder(this, CHANNEL_ID)
-            .setSmallIcon(android.R.drawable.ic_lock_idle_alarm)
+            .setSmallIcon(R.drawable.ic_stat_pluriwave)
+            .setColor(NotificationBrand.CYAN)
             .setContentTitle(AlarmNotificationStrings.ringTitle(this))
             .setContentText(
                 if (stationName.isNullOrBlank()) title else "$title - $stationName"

android/app/src/main/res/drawable/ic_stat_pluriwave.xml

@@ -0,0 +1,3 @@
+<vector xmlns:android="http://schemas.android.com/apk/res/android" android:width="24dp" android:height="24dp" android:viewportWidth="24" android:viewportHeight="24">
+  <path android:fillColor="#FFFFFF" android:pathData="M7,18h2L9,6L7,6v12zM3,14h2v-4L3,10v4zM11,20h2L13,4h-2v16zM19,10v4h2v-4h-2zM15,18h2L17,6h-2v12z" />
+</vector>