feat(io): MusicLevel enum (Off/Low/Med/High)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Semesterprojekt/src/main/java/thb/jeanluc/adventure/io/MusicLevel.java

@@ -0,0 +1,26 @@
+package thb.jeanluc.adventure.io;
+
+/** Background-music volume level, cycled in the Settings menu. */
+public enum MusicLevel {
+    OFF, LOW, MEDIUM, HIGH;
+
+    /** Target MASTER_GAIN in decibels. Irrelevant for {@link #OFF} (playback disabled). */
+    public float gainDb() {
+        return switch (this) {
+            case OFF -> Float.NEGATIVE_INFINITY;
+            case LOW -> -20f;
+            case MEDIUM -> -10f;
+            case HIGH -> 0f;
+        };
+    }
+
+    /** @return true when music should play. */
+    public boolean isOn() {
+        return this != OFF;
+    }
+
+    /** Next level in the cycle (wraps HIGH → OFF). */
+    public MusicLevel next() {
+        return values()[(ordinal() + 1) % values().length];
+    }
+}