Add video proxy support with live toggle for Invidious/Piped instances

Adds a "Proxy videos" toggle in instance settings that routes video
streams through the instance instead of connecting directly to YouTube
CDN. Includes auto-detection of 403 blocks and live re-application of
proxy settings without requiring app restart or video reload.

Yattee/Services/Player/PlayerState.swift

@@ -451,6 +451,16 @@ final class PlayerState {
         }
     }
 
+    /// Updates the current stream without changing the video.
+    func updateCurrentStream(_ stream: Stream) {
+        currentStream = stream
+    }
+
+    /// Updates the current audio stream without changing the video.
+    func updateCurrentAudioStream(_ stream: Stream) {
+        currentAudioStream = stream
+    }
+
     /// Whether the playback has failed.
     var isFailed: Bool {
         if case .failed = playbackState { return true }