feat: show last activity timestamp per device on Tuerschilder page

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

templates/devices.html

@@ -29,6 +29,7 @@
                                 <th>Name</th>
                                 <th>LEDs</th>
                                 <th>Version</th>
+                                <th>Letzte Aktivitaet</th>
                                 <th>Segmente</th>
                                 <th>Aktionen</th>
                             </tr>
@@ -53,6 +54,7 @@
                                 <td>{{ device.name }}</td>
                                 <td>{{ device.leds if device.leds else '-' }}</td>
                                 <td>{{ device.version if device.version else '-' }}</td>
+                                <td><small>{{ device.last_activity or '-' }}</small></td>
                                 <td>{{ device.segment_count }}</td>
                                 <td>
                                     <form method="post" action="/api/devices/{{ device.id }}/check" class="d-inline">
@@ -112,7 +114,7 @@
 
                             {% if not devices %}
                             <tr>
-                                <td colspan="7" class="text-center text-muted py-4">
+                                <td colspan="8" class="text-center text-muted py-4">
                                     Noch keine Tuerschilder vorhanden.
                                 </td>
                             </tr>