Add background notifications and fix Discord/Slack notification bugs

Background Notification Features:
- Implement Web Push Notifications for alerts when browser is minimized
- Add Service Worker for persistent background notification support
- Integrate Page Visibility API to detect when page is in background
- Add browser notification permission request with user consent flow
- Create more aggressive background polling (10 seconds) when page hidden
- Add vibration patterns for mobile device alerts

Bug Fixes:
- Fix Discord/Slack notification parameter order bug preventing delivery
- Add comprehensive logging for notification debugging
- Correct webhook function signatures for proper data passing

Mobile Enhancements:
- Support system notifications on Android browsers
- Add click-to-focus functionality for notifications
- Implement background alert system for multitasking
- Add notification button with visual feedback

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

includes/class-twp-call-queue.php

@@ -562,6 +562,8 @@ class TWP_Call_Queue {
     private static function notify_agents_for_queue($queue_id, $caller_number) {
         global $wpdb;
         
+        error_log("TWP: notify_agents_for_queue called for queue {$queue_id}, caller {$caller_number}");
+        
         // Get queue information including assigned agent group and phone number
         $queue_table = $wpdb->prefix . 'twp_call_queues';
         $queue = $wpdb->get_row($wpdb->prepare(
@@ -569,13 +571,21 @@ class TWP_Call_Queue {
             $queue_id
         ));
         
-        if (!$queue || !$queue->agent_group_id) {
+        if (!$queue) {
+            error_log("TWP: Queue {$queue_id} not found in database");
+            return;
+        }
+        
+        if (!$queue->agent_group_id) {
             error_log("TWP: No agent group assigned to queue {$queue_id}, skipping SMS notifications");
             return;
         }
         
+        error_log("TWP: Found queue '{$queue->queue_name}' with agent group {$queue->agent_group_id}");
+        
         // Send Discord/Slack notification for incoming call
         require_once dirname(__FILE__) . '/class-twp-notifications.php';
+        error_log("TWP: Triggering Discord/Slack notification for incoming call");
         TWP_Notifications::send_call_notification('incoming_call', array(
             'type' => 'incoming_call',
             'caller' => $caller_number,