v3.7.0: roll back from v4, remove internal player — v3 is already complete

Version rolled back from v4.x to v3.7.0. After extensive work on v4's
internal streaming engine, smart queue, DASH pipeline, and media controls,
we realized v3 was already feature-complete. Adding more big features
only made maintenance increasingly difficult and the developer's life
miserable. Stripped back to what works: external player only, cleaner
codebase, sustainable long-term.

- Remove just_audio, audio_service, audio_session and entire internal
  playback engine (smart queue, notification, shuffle/repeat, prefetch)
- Remove PlaybackItem model, MiniPlayerBar widget, notification drawables
- Remove playerMode setting (external-only now)
- Migrate MainActivity from AudioServiceFragmentActivity to
  FlutterFragmentActivity
- Migrate Qobuz to MusicDL API
- Update changelog with v3.7.0 rollback explanation

android/app/proguard-rules.pro

@@ -80,16 +80,6 @@
 -keep class io.flutter.plugins.pathprovider.** { *; }
 -keep class dev.flutter.pigeon.** { *; }
 
-# Audio Service (media playback notification) - CRITICAL for release builds
--keep class com.ryanheise.audioservice.** { *; }
--keep class com.ryanheise.audio_session.** { *; }
--keep class com.ryanheise.just_audio.** { *; }
-
-# AndroidX Media / MediaSession (used by audio_service)
--keep class androidx.media.** { *; }
--keep class android.support.v4.media.** { *; }
--dontwarn android.support.v4.media.**
-
 # Local Notifications
 -keep class com.dexterous.** { *; }
 -keep class com.dexterous.flutterlocalnotifications.** { *; }

android/app/src/main/kotlin/com/zarz/spotiflac/MainActivity.kt

@@ -7,7 +7,7 @@ import android.os.Build
 import androidx.activity.OnBackPressedCallback
 import androidx.activity.result.contract.ActivityResultContracts
 import androidx.documentfile.provider.DocumentFile
-import com.ryanheise.audioservice.AudioServiceFragmentActivity
+import io.flutter.embedding.android.FlutterFragmentActivity
 import io.flutter.embedding.android.FlutterActivityLaunchConfigs.BackgroundMode
 import io.flutter.embedding.android.FlutterFragment
 import io.flutter.embedding.android.RenderMode
@@ -32,7 +32,7 @@ import java.io.FileInputStream
 import java.io.FileOutputStream
 import java.util.Locale
 
-class MainActivity: AudioServiceFragmentActivity() {
+class MainActivity: FlutterFragmentActivity() {
     private val CHANNEL = "com.zarz.spotiflac/backend"
     private val DOWNLOAD_PROGRESS_STREAM_CHANNEL =
         "com.zarz.spotiflac/download_progress_stream"
@@ -50,6 +50,7 @@ class MainActivity: AudioServiceFragmentActivity() {
     private var libraryScanProgressStreamJob: Job? = null
     private var libraryScanProgressEventSink: EventChannel.EventSink? = null
     private var lastLibraryScanProgressPayload: String? = null
+    private var flutterBackCallback: OnBackPressedCallback? = null
     @Volatile private var safScanCancel = false
     @Volatile private var safScanActive = false
     private val safTreeLauncher = registerForActivityResult(
@@ -1370,11 +1371,12 @@ class MainActivity: AudioServiceFragmentActivity() {
         // which disables Flutter's own OnBackPressedCallback and causes the
         // system default (finish activity) to run. This callback guarantees
         // popRoute is always forwarded to Flutter, where PopScope handles it.
-        onBackPressedDispatcher.addCallback(this, object : OnBackPressedCallback(true) {
+        flutterBackCallback = object : OnBackPressedCallback(true) {
             override fun handleOnBackPressed() {
                 flutterEngine.navigationChannel.popRoute()
             }
-        })
+        }
+        onBackPressedDispatcher.addCallback(this, flutterBackCallback!!)
 
         val messenger = flutterEngine.dartExecutor.binaryMessenger
 
@@ -1416,6 +1418,15 @@ class MainActivity: AudioServiceFragmentActivity() {
             scope.launch {
                 try {
                     when (call.method) {
+                        "exitApp" -> {
+                            flutterBackCallback?.isEnabled = false
+                            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
+                                finishAndRemoveTask()
+                            } else {
+                                finish()
+                            }
+                            result.success(null)
+                        }
                         "parseSpotifyUrl" -> {
                             val url = call.argument<String>("url") ?: ""
                             val response = withContext(Dispatchers.IO) {

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

@@ -1,9 +0,0 @@
-<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="#FFFFFFFF"
-        android:pathData="M12,21.35l-1.45,-1.32C5.4,15.36 2,12.28 2,8.5 2,5.42 4.42,3 7.5,3c1.74,0 3.41,0.81 4.5,2.09C13.09,3.81 14.76,3 16.5,3 19.58,3 22,5.42 22,8.5c0,3.78 -3.4,6.86 -8.55,11.54L12,21.35z"/>
-</vector>

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

@@ -1,9 +0,0 @@
-<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="#FFFFFFFF"
-        android:pathData="M16.5,3C14.76,3 13.09,3.81 12,5.09C10.91,3.81 9.24,3 7.5,3C4.42,3 2,5.42 2,8.5C2,12.28 5.4,15.36 10.55,20.03L12,21.35L13.45,20.03C18.6,15.36 22,12.28 22,8.5C22,5.42 19.58,3 16.5,3ZM12.1,18.55L12,18.65L11.9,18.55C7.14,14.24 4,11.39 4,8.5C4,6.5 5.5,5 7.5,5C9.04,5 10.54,5.99 11.07,7.36H12.94C13.46,5.99 14.96,5 16.5,5C18.5,5 20,6.5 20,8.5C20,11.39 16.86,14.24 12.1,18.55Z"/>
-</vector>

android/app/src/main/res/raw/keep.xml

@@ -1,3 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<resources xmlns:tools="http://schemas.android.com/tools"
-    tools:keep="@drawable/ic_stat_favorite,@drawable/ic_stat_favorite_border" />