Add an Agent Version column to the Fleet view

Placed between Health and Logged-in User on /endpoints. The main agent
version, not the tray: both ship side by side in the payload as
agentversion and trayversion, so the wrong one is an easy pick.

Frontend only. EndpointState.to_dict() has carried agentversion all
along, so the fleet list API was already sending the field and nothing
rendered it.

Also bumped the empty-state colspan from 9/8 to 10/9. It is hardcoded
rather than derived from the header count, so it does not track a new
column on its own and silently under-spans the "No endpoints found" row
if missed.

plugins/endpoints/frontend/views/EndpointsList.vue

@@ -91,6 +91,7 @@
                 <th>Machine</th>
                 <th>PC Type</th>
                 <th>Health</th>
+                <th>Agent Version</th>
                 <th>Logged-in User</th>
                 <th>Uptime</th>
                 <th>Last Reported</th>
@@ -123,12 +124,13 @@
                         class="health-pill" :class="item.health.level">{{ item.health.label }}</span>
                   <span v-else>-</span>
                 </td>
+                <td>{{ item.agentversion || '-' }}</td>
                 <td>{{ item.loggedinuser || '-' }}</td>
                 <td>{{ item.uptimedisplay || '-' }}</td>
                 <td>{{ formatRelative(item.lastreportedat) }}</td>
               </tr>
               <tr v-if="endpoints.length === 0">
-                <td :colspan="cancommand ? 9 : 8" style="text-align: center; color: var(--text-light);">
+                <td :colspan="cancommand ? 10 : 9" style="text-align: center; color: var(--text-light);">
                   No endpoints found
                 </td>
               </tr>