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/Migration/LegacyDataMigrationService.swift

@@ -84,7 +84,8 @@ final class LegacyDataMigrationService {
                 legacyInstanceID: instance.id,
                 instanceType: instanceType,
                 url: url,
-                name: instance.name.isEmpty ? nil : instance.name
+                name: instance.name.isEmpty ? nil : instance.name,
+                proxiesVideos: instance.proxiesVideos
             )
             items.append(item)
         }
@@ -184,7 +185,8 @@ final class LegacyDataMigrationService {
             type: item.instanceType,
             url: item.url,
             name: item.name,
-            isEnabled: true
+            isEnabled: true,
+            proxiesVideos: item.proxiesVideos
         )
 
         // Add to instances manager

Yattee/Services/Migration/LegacyDataModels.swift

@@ -99,6 +99,9 @@ struct LegacyImportItem: Identifiable, Sendable {
     /// User-defined name for the instance
     let name: String?
 
+    /// Whether this instance proxies videos
+    var proxiesVideos: Bool = false
+
     /// Whether this item is selected for import
     var isSelected: Bool = true