fix: Certificate viewing URLs and duplicate prevention display

This commit addresses the final two certificate issues reported by the user:

## 1. Fixed Certificate View 404 Error
**Issue:** Clicking 'View' on certificates resulted in 404 error at /hvac-certificate/{token}/ URLs
**Solution:**
- The certificate security system uses custom rewrite rules for secure download URLs
- Added rewrite rule: hvac-certificate/([^/]+)/?$ → index.php?certificate_token=
- WordPress rewrite rules were not properly flushed after plugin updates
- Deploy script now ensures rewrite rules are flushed on every deployment
- This enables the secure token-based certificate download system to function properly

## 2. Enhanced Certificate Duplicate Prevention Display
**Issue:** Users received confusing 'Failed to generate certificates. 1 duplicate(s) skipped.' message
**Solution:**
- Added certificate status checking in Generate Certificates template
- New 'Certificate Status' column shows which attendees already have certificates
- Attendees with existing certificates:
  - Show checkmark (✓) instead of checkbox
  - Display 'Certificate Issued' status
  - Cannot be selected for regeneration
  - Are visually distinguished with styling
- Added informative notice explaining duplicate prevention
- Users now see exactly which attendees have certificates before attempting generation

## Technical Implementation:
- Certificate security handler properly validates tokens and serves PDFs
- One-time use tokens with 1-hour expiry for security
- Certificate manager checks for existing certificates per attendee
- UI clearly indicates certificate status to prevent confusion

These fixes complete the certificate functionality restoration, providing a smooth user experience for certificate generation, viewing, and management.

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

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

wordpress-dev/wordpress/wp-content/plugins/hvac-community-events/templates/certificates/template-generate-certificates.php

@@ -72,6 +72,14 @@ try {
                 $event_id
             ));
             
+            // Check certificate status for each attendee
+            if (!empty($attendees) && class_exists('HVAC_Certificate_Manager')) {
+                $certificate_manager = HVAC_Certificate_Manager::instance();
+                foreach ($attendees as $attendee) {
+                    $attendee->has_certificate = $certificate_manager->certificate_exists($event_id, $attendee->attendee_id);
+                }
+            }
+            
             // Log for debugging if needed
             if (defined('WP_DEBUG') && WP_DEBUG) {
                 error_log('Generate Certificates - Event ID: ' . $event_id . ', Attendees: ' . count($attendees));
@@ -143,6 +151,22 @@ get_header();
                         <input type="hidden" name="event_id" value="<?php echo esc_attr($event_id); ?>">
                         <input type="hidden" name="generate_certificates" value="1">
                         
+                        <?php 
+                        // Count attendees with certificates
+                        $has_certificate_count = 0;
+                        foreach ($attendees as $attendee) {
+                            if (!empty($attendee->has_certificate)) {
+                                $has_certificate_count++;
+                            }
+                        }
+                        
+                        if ($has_certificate_count > 0) : ?>
+                            <div class="hvac-notice hvac-notice-info">
+                                <p><strong>Note:</strong> <?php echo $has_certificate_count; ?> attendee(s) already have certificates. These will be skipped to prevent duplicates.</p>
+                                <p>Attendees with existing certificates are marked with ✓ and cannot be selected.</p>
+                            </div>
+                        <?php endif; ?>
+                        
                         <div class="hvac-form-group">
                             <div class="hvac-table-actions">
                                 <button type="button" class="hvac-button hvac-secondary" id="select-all-attendees">Select All</button>
@@ -160,6 +184,7 @@ get_header();
                                             <th>Attendee Name</th>
                                             <th>Email</th>
                                             <th>Check-in Status</th>
+                                            <th>Certificate Status</th>
                                         </tr>
                                     </thead>
                                     <tbody>
@@ -170,14 +195,20 @@ get_header();
                                             $status_text = $checked_in ? 'Checked In' : 'Not Checked In';
                                             $attendee_name = $attendee->holder_name ?: 'Unknown';
                                             $attendee_email = $attendee->holder_email ?: 'No email';
+                                            $has_certificate = !empty($attendee->has_certificate);
+                                            $cert_status_class = $has_certificate ? 'hvac-has-certificate' : '';
                                         ?>
-                                            <tr class="<?php echo esc_attr($checked_in_class); ?>">
+                                            <tr class="<?php echo esc_attr($checked_in_class . ' ' . $cert_status_class); ?>">
                                                 <td>
+                                                    <?php if (!$has_certificate) : ?>
                                                         <input type="checkbox" 
                                                                name="attendee_ids[]" 
                                                                value="<?php echo esc_attr($attendee->attendee_id); ?>" 
                                                                class="attendee-checkbox" 
                                                                <?php echo $checked_in ? 'checked' : ''; ?>>
+                                                    <?php else : ?>
+                                                        <span class="hvac-certificate-exists" title="Certificate already generated">✓</span>
+                                                    <?php endif; ?>
                                                 </td>
                                                 <td><?php echo esc_html($attendee_name); ?></td>
                                                 <td><?php echo esc_html($attendee_email); ?></td>
@@ -186,6 +217,13 @@ get_header();
                                                         <?php echo esc_html($status_text); ?>
                                                     </span>
                                                 </td>
+                                                <td>
+                                                    <?php if ($has_certificate) : ?>
+                                                        <span class="hvac-status-has-certificate">Certificate Issued</span>
+                                                    <?php else : ?>
+                                                        <span class="hvac-status-no-certificate">No Certificate</span>
+                                                    <?php endif; ?>
+                                                </td>
                                             </tr>
                                         <?php endforeach; ?>
                                     </tbody>