feat(player): add beat-matched AutoMix and refine Mornye navigation

Add default-off local beat analysis, pitch-preserving tempo matching, crossfade fallback, and cancellation-safe two-deck playback. Expose AutoMix in settings and the Mornye queue.

Separate Search from Home, remove the Mornye greeting, and keep Home/Search icons moving continuously between full and compact navigation. Restore expansion on upward Library scrolling.

Validate beat analysis, transport handoff and cancellation, settings persistence, and navigation behavior. Flutter analysis and the signed iOS simulator build pass.

lib/models/settings.dart

@@ -31,6 +31,7 @@ class AppSettings {
   final bool embedReplayGain;
   // Apply ReplayGain/R128 tags as volume normalization in the built-in player.
   final bool playbackNormalization;
+  final bool autoMix;
   final bool discordRichPresence;
   final bool isFirstLaunch;
   final bool checkForUpdates;
@@ -139,6 +140,7 @@ class AppSettings {
     this.embedLyrics = true,
     this.embedReplayGain = false,
     this.playbackNormalization = false,
+    this.autoMix = false,
     this.discordRichPresence = false,
     this.isFirstLaunch = true,
     this.checkForUpdates = true,
@@ -223,6 +225,7 @@ class AppSettings {
     bool? embedLyrics,
     bool? embedReplayGain,
     bool? playbackNormalization,
+    bool? autoMix,
     bool? discordRichPresence,
     bool? isFirstLaunch,
     bool? checkForUpdates,
@@ -303,6 +306,7 @@ class AppSettings {
       embedReplayGain: embedReplayGain ?? this.embedReplayGain,
       playbackNormalization:
           playbackNormalization ?? this.playbackNormalization,
+      autoMix: autoMix ?? this.autoMix,
       discordRichPresence: discordRichPresence ?? this.discordRichPresence,
       isFirstLaunch: isFirstLaunch ?? this.isFirstLaunch,
       checkForUpdates: checkForUpdates ?? this.checkForUpdates,

lib/models/settings.g.dart

@@ -22,6 +22,7 @@ AppSettings _$AppSettingsFromJson(Map<String, dynamic> json) => AppSettings(
   embedLyrics: json['embedLyrics'] as bool? ?? true,
   embedReplayGain: json['embedReplayGain'] as bool? ?? false,
   playbackNormalization: json['playbackNormalization'] as bool? ?? false,
+  autoMix: json['autoMix'] as bool? ?? false,
   discordRichPresence: json['discordRichPresence'] as bool? ?? false,
   isFirstLaunch: json['isFirstLaunch'] as bool? ?? true,
   checkForUpdates: json['checkForUpdates'] as bool? ?? true,
@@ -118,6 +119,7 @@ Map<String, dynamic> _$AppSettingsToJson(
   'embedLyrics': instance.embedLyrics,
   'embedReplayGain': instance.embedReplayGain,
   'playbackNormalization': instance.playbackNormalization,
+  'autoMix': instance.autoMix,
   'discordRichPresence': instance.discordRichPresence,
   'isFirstLaunch': instance.isFirstLaunch,
   'checkForUpdates': instance.checkForUpdates,