Add optional AVPlayer support for non-streamable MP4/AVC1 formats

AVPlayer has a fundamental limitation with MP4/AVC1 progressive downloads where the moov atom position affects playback start time. When moov is at the end of the file, AVPlayer must download the entire file before starting playback. MPV doesn't have this limitation.

This commit adds an advanced setting to optionally enable these formats in AVPlayer with appropriate warnings:

- Added new setting: "Enable non-streamable formats (MP4/AVC1)"
- Default: disabled (formats hidden, MPV handles them)
- When enabled: MP4/AVC1 formats up to 1080p appear in AVPlayer quality selector
- Resolution limit: 1080p maximum (higher resolutions can't be played properly)
- Clear warning about slow loading and 1080p limitation
- Automatic stream refresh when setting is toggled
- Full import/export support for the setting

Model/Player/Backends/AVPlayerBackend.swift

@@ -11,6 +11,8 @@ import SwiftUI
 final class AVPlayerBackend: PlayerBackend {
     static let assetKeysToLoad = ["tracks", "playable", "duration"]
 
+    @Default(.avPlayerAllowsNonStreamableFormats) private var allowsNonStreamableFormats
+
     private var logger = Logger(label: "avplayer-backend")
 
     var model: PlayerModel { .shared }
@@ -150,7 +152,30 @@ final class AVPlayerBackend: PlayerBackend {
     }
 
     func canPlay(_ stream: Stream) -> Bool {
-        stream.kind == .hls || stream.kind == .stream
+        // AVPlayer has a fundamental limitation with MP4/AVC1 progressive downloads:
+        // If the moov atom is at the end of the file (common case), it must download
+        // the entire file before playback can start. MPV doesn't have this limitation.
+        // By default, reject non-HLS MP4/AVC1 streams unless user explicitly enables them.
+
+        // Check if this is a non-streamable format (MP4/AVC1) that isn't HLS
+        let isNonStreamableFormat = stream.kind != .hls && (stream.format == .mp4 || stream.format == .avc1)
+
+        if isNonStreamableFormat && !allowsNonStreamableFormats {
+            return false
+        }
+
+        // If non-streamable formats are enabled, allow MP4/AVC1 adaptive streams
+        // but limit to 1080p maximum (higher resolutions can't be played properly)
+        if isNonStreamableFormat && allowsNonStreamableFormats {
+            let maxHeight = 1080
+            if let resolution = stream.resolution, resolution.height > maxHeight {
+                return false
+            }
+            return true
+        }
+
+        // AVPlayer works well with HLS and stream formats
+        return stream.kind == .hls || stream.kind == .stream
     }
 
     func playStream(
@@ -304,12 +329,12 @@ final class AVPlayerBackend: PlayerBackend {
         preservingTime: Bool = false,
         model: PlayerModel
     ) {
+        model.logger.info("loading \(type.rawValue) track")
+
         asset.loadValuesAsynchronously(forKeys: Self.assetKeysToLoad) { [weak self] in
             guard let self else {
                 return
             }
-            model.logger.info("loading \(type.rawValue) track")
-
             let assetTracks = asset.tracks(withMediaType: type)
 
             guard let compositionTrack = self.composition.addMutableTrack(