Audit quick wins: null guard, API error toasts, aria-labels on suppress buttons

- app.js: guard events array with || [] before .filter() to prevent crash on null
- app.js: show warning toast when /api/network or /api/status fail (was silent)
- app.js: add aria-label to all dynamically-generated suppress buttons
- index.html: add aria-label to server-rendered suppress buttons

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/index.html

@@ -336,7 +336,7 @@
                       data-sup-type="{{ 'unifi_device' if e.event_type == 'unifi_device_offline' else 'interface' if e.event_type == 'interface_down' else 'host' }}"
                       data-sup-name="{{ e.target_name }}"
                       data-sup-detail="{{ e.target_detail or '' }}"
-                      title="Suppress">🔕</button>
+                      title="Suppress" aria-label="Suppress alert for {{ e.target_name }}">🔕</button>
             </td>
           </tr>
           {% endif %}