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

pubspec.lock

@@ -49,38 +49,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.13.0"
-  audio_service:
-    dependency: "direct main"
-    description:
-      name: audio_service
-      sha256: cb122c7c2639d2a992421ef96b67948ad88c5221da3365ccef1031393a76e044
-      url: "https://pub.dev"
-    source: hosted
-    version: "0.18.18"
-  audio_service_platform_interface:
-    dependency: transitive
-    description:
-      name: audio_service_platform_interface
-      sha256: "6283782851f6c8b501b60904a32fc7199dc631172da0629d7301e66f672ab777"
-      url: "https://pub.dev"
-    source: hosted
-    version: "0.1.3"
-  audio_service_web:
-    dependency: transitive
-    description:
-      name: audio_service_web
-      sha256: b8ea9243201ee53383157fbccf13d5d2a866b5dda922ec19d866d1d5d70424df
-      url: "https://pub.dev"
-    source: hosted
-    version: "0.1.4"
-  audio_session:
-    dependency: "direct main"
-    description:
-      name: audio_session
-      sha256: "8f96a7fecbb718cb093070f868b4cdcb8a9b1053dce342ff8ab2fde10eb9afb7"
-      url: "https://pub.dev"
-    source: hosted
-    version: "0.2.2"
   boolean_selector:
     dependency: transitive
     description:
@@ -613,30 +581,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "6.11.2"
-  just_audio:
-    dependency: "direct main"
-    description:
-      name: just_audio
-      sha256: "9694e4734f515f2a052493d1d7e0d6de219ee0427c7c29492e246ff32a219908"
-      url: "https://pub.dev"
-    source: hosted
-    version: "0.10.5"
-  just_audio_platform_interface:
-    dependency: transitive
-    description:
-      name: just_audio_platform_interface
-      sha256: "2532c8d6702528824445921c5ff10548b518b13f808c2e34c2fd54793b999a6a"
-      url: "https://pub.dev"
-    source: hosted
-    version: "4.6.0"
-  just_audio_web:
-    dependency: transitive
-    description:
-      name: just_audio_web
-      sha256: "6ba8a2a7e87d57d32f0f7b42856ade3d6a9fbe0f1a11fabae0a4f00bb73f0663"
-      url: "https://pub.dev"
-    source: hosted
-    version: "0.4.16"
   leak_tracker:
     dependency: transitive
     description: