Add swingmusic-mobile submodule to replace Android app

- Updated .gitmodules to include mobile app submodule
- Added swingmusic_mobile directory with Flutter app
- Mobile app will now be built in unified release workflow

swingmusic_mobile/lib/core/enums/auth_state.dart

@@ -0,0 +1,24 @@
+enum AuthState {
+  loggedOut,
+  authenticating,
+  authenticated,
+  error;
+
+  String get displayName {
+    switch (this) {
+      case AuthState.loggedOut:
+        return 'Logged Out';
+      case AuthState.authenticating:
+        return 'Authenticating';
+      case AuthState.authenticated:
+        return 'Authenticated';
+      case AuthState.error:
+        return 'Error';
+    }
+  }
+
+  bool get isLoggedIn => this == AuthState.authenticated;
+  bool get isLoggedOut => this == AuthState.loggedOut;
+  bool get isAuthenticating => this == AuthState.authenticating;
+  bool get hasError => this == AuthState.error;
+}

swingmusic_mobile/lib/core/enums/playback_mode.dart

@@ -0,0 +1,50 @@
+enum RepeatMode {
+  off,
+  one,
+  all;
+
+  String get displayName {
+    switch (this) {
+      case RepeatMode.off:
+        return 'Off';
+      case RepeatMode.one:
+        return 'Repeat One';
+      case RepeatMode.all:
+        return 'Repeat All';
+    }
+  }
+
+  RepeatMode next() {
+    switch (this) {
+      case RepeatMode.off:
+        return RepeatMode.all;
+      case RepeatMode.all:
+        return RepeatMode.one;
+      case RepeatMode.one:
+        return RepeatMode.off;
+    }
+  }
+}
+
+enum ShuffleMode {
+  off,
+  on;
+
+  String get displayName {
+    switch (this) {
+      case ShuffleMode.off:
+        return 'Off';
+      case ShuffleMode.on:
+        return 'On';
+    }
+  }
+
+  ShuffleMode toggle() {
+    switch (this) {
+      case ShuffleMode.off:
+        return ShuffleMode.on;
+      case ShuffleMode.on:
+        return ShuffleMode.off;
+    }
+  }
+}