Remove Enable DASH setting; make DASH a last-resort format

Drop the dashEnabled toggle and its plumbing. Stream auto-selection now
keeps DASH as a candidate but ranks it strictly below progressive and
HLS formats, so DASH is only chosen when it is the only format available
(including live-stream fallback after HLS). The manual quality selector
hides DASH entirely on iOS/tvOS/macOS. Reconciles BackendSwitcher, which
previously treated HLS/DASH equally, to use the same DASH-last ranking.

Yattee/Core/Settings/SettingsKey.swift

@@ -20,7 +20,6 @@ enum SettingsKey: String, CaseIterable {
     case cellularQuality
     case autoplay
     case backgroundPlayback
-    case dashEnabled
     case preferredAudioLanguage
     case preferredSubtitlesLanguage
     case resumeAction

Yattee/Core/Settings/SettingsManager+Playback.swift

@@ -70,19 +70,6 @@ extension SettingsManager {
         }
     }
 
-    /// Whether DASH streams are enabled (MPV only).
-    /// Disabled by default as DASH can be unreliable with some Invidious instances.
-    var dashEnabled: Bool {
-        get {
-            if let cached = _dashEnabled { return cached }
-            return bool(for: .dashEnabled, default: false)
-        }
-        set {
-            _dashEnabled = newValue
-            set(newValue, for: .dashEnabled)
-        }
-    }
-
     var allowSoftwareDecodedFormats: Bool {
         get {
             if let cached = _allowSoftwareDecodedFormats { return cached }