Fix feature flag preference decoding with old flags

Packages/TuskerPreferences/Sources/TuskerPreferences/Keys/AdvancedKeys.swift

@@ -12,6 +12,17 @@ struct StatusContentTypeKey: MigratablePreferenceKey {
     static var defaultValue: StatusContentType { .plain }
 }
 
-struct FeatureFlagsKey: MigratablePreferenceKey {
+struct FeatureFlagsKey: MigratablePreferenceKey, CustomCodablePreferenceKey {
     static var defaultValue: Set<FeatureFlag> { [] }
+    
+    static func encode(value: Set<FeatureFlag>, to encoder: any Encoder) throws {
+        var container = encoder.singleValueContainer()
+        try container.encode(value.map(\.rawValue))
+    }
+    
+    static func decode(from decoder: any Decoder) throws -> Set<FeatureFlag>? {
+        let container = try decoder.singleValueContainer()
+        let names = try container.decode([String].self)
+        return Set(names.compactMap(FeatureFlag.init(rawValue:)))
+    }
 }