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/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" />