chore(android): TEMP nav back-stack debug strip (diagnosing back→home bug)

Shows the live NavController back stack at the top of the screen so we can see
whether detail screens are being pushed correctly or the back button isn't being
handled. To be reverted once the back-navigation issue is diagnosed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/ui/AppNavigation.kt

@@ -395,6 +395,21 @@ fun AppNavigation() {
             }
         }
 
+        // ── TEMP NAV DEBUG — zeigt den Back-Stack; wird nach der Diagnose entfernt ──
+        val dbgBackStack by navController.currentBackStack.collectAsStateWithLifecycle()
+        Box(Modifier.fillMaxWidth().statusBarsPadding(), contentAlignment = androidx.compose.ui.Alignment.TopCenter) {
+            Text(
+                "🧭 " + dbgBackStack.mapNotNull { it.destination.route?.substringBefore("/") }
+                    .filter { it != "null" }.joinToString(" › "),
+                color = Color(0xFFFFEB3B),
+                fontSize = 9.sp,
+                fontWeight = FontWeight.Bold,
+                modifier = Modifier
+                    .background(Color(0xCC000000))
+                    .padding(horizontal = 6.dp, vertical = 2.dp),
+            )
+        }
+
         // NowPlaying overlay
         AnimatedVisibility(
             visible = showNowPlaying,