fix: restore MainActor context for notification observation task

[sozercan]

Summary

• Adds @MainActor annotation to the Task closure in NotificationService.startObserving()
• Fixes notification observation that broke when nonisolated(unsafe) was added in commit 6f0602d

Root Cause

When nonisolated(unsafe) was added to observationTask to fix Swift 6 deinit warnings, the Task lost its implicit @MainActor context inheritance. This caused the polling loop to not properly observe changes to the @MainActor-isolated playerService.currentTrack property.

Fix

Adding @MainActor to the Task closure ensures proper actor isolation, matching the pattern already used in NetworkMonitor.

// Before (broken):
self.observationTask = Task { [weak self] in

// After (fixed):
self.observationTask = Task { @MainActor [weak self] in

Test plan

• Play a track and verify notification appears
• Skip to next track and verify new notification appears
• Toggle "Show Now Playing Notifications" setting and verify it's respected

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a bug in notification observation that was inadvertently broken when nonisolated(unsafe) was added to observationTask to resolve Swift 6 deinit warnings. The fix restores proper actor isolation by adding @MainActor to the Task closure, ensuring the polling loop can correctly observe changes to @MainActor-isolated properties.

Changes:

• Added @MainActor annotation to the Task closure in NotificationService.startObserving()

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