Register UNNotificationCategory for native iOS notification actions + Apple Watch support

Summary

Action buttons currently only appear inside the ntfy app. By registering UNNotificationCategory with UNNotificationAction, buttons would appear as native iOS notification actions AND automatically mirror to Apple Watch.

Current Behavior

• Action buttons: ✅ Visible in ntfy app message detail view
• Action buttons: ❌ NOT visible in iOS Notification Center (swipe/long-press)
• Action buttons: ❌ NOT visible on Apple Watch (only "Dismiss" shown)

Expected Behavior

When a notification with actions arrives:

1. Swiping/long-pressing the notification in iOS shows action buttons
2. Apple Watch shows action buttons (up to 4, first 2 in compact view)
3. Tapping an action triggers the HTTP request directly from notification

Technical Approach

1. Register UNNotificationCategory for notifications containing actions
2. Create UNNotificationAction instances for each action (view, http)
3. Include the category identifier in the push payload
4. iOS will display actions in Notification Center
5. watchOS will automatically mirror actions without needing a watchOS app

Why This Matters

• Speed: Respond to notifications without opening the app
• Apple Watch: Huge UX improvement - respond from wrist
• Consistency: Match Android behavior where actions appear in notification shade

References

• Apple: Declaring actionable notification types
• Apple: Adding actions to notifications on watchOS
• Related: #1156 (action button clear not working - may be connected)