Keep Sparkle resetUpdateCycle on the main actor

SPUUpdater is a main-thread-only API; calling resetUpdateCycle() from a
detached utility task violated Sparkle's threading contract and raised
a Swift 6 concurrency error (non-Sendable SPUUpdater captured in a
detached task). A plain main-actor Task keeps the deferral that avoids
running the feed-cache work synchronously inside the didSet.

Yattee/Services/Updates/SparkleUpdater.swift

@@ -33,12 +33,12 @@ final class AppUpdater {
         didSet {
             guard oldValue != wantsBetaChannel else { return }
             UserDefaults.standard.set(wantsBetaChannel, forKey: Self.wantsBetaKey)
-            // Trigger re-check so the delegate re-reads channels. Deferred off
-            // the main thread — resetUpdateCycle may synchronously touch the
-            // scheduler's feed cache which can stutter SwiftUI scrolling.
-            let updater = self.updaterController.updater
-            Task.detached(priority: .utility) {
-                updater.resetUpdateCycle()
+            // Trigger re-check so the delegate re-reads channels. Deferred to
+            // a later main-actor turn so the feed-cache work doesn't run
+            // synchronously inside this didSet (which can stutter SwiftUI
+            // scrolling). Must stay on main: SPUUpdater is main-thread-only.
+            Task { @MainActor in
+                self.updaterController.updater.resetUpdateCycle()
             }
         }
     }