feat: display semantic version in sidebar footer

Reads version from package.json via Vite's define (__APP_VERSION__)
and shows it as a small label at the bottom of the sidebar.

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

frontend/src/App.vue

@@ -85,6 +85,7 @@
             </svg>
           </button>
         </div>
+        <div class="app-version">v{{ appVersion }}</div>
       </div>
     </aside>
 
@@ -127,6 +128,8 @@ import DiscoveryModal from './components/DiscoveryModal.vue'
 import LoginPage from './components/LoginPage.vue'
 import AccountModal from './components/AccountModal.vue'
 
+const appVersion = __APP_VERSION__
+
 const view = ref('topology')
 const vlans = ref([])
 const devices = ref([])
@@ -454,6 +457,14 @@ nav {
 }
 .logout-btn:hover { background: #7F1D1D; color: #FCA5A5; }
 
+.app-version {
+  text-align: center;
+  font-size: 10px;
+  color: #475569;
+  margin-top: 6px;
+  letter-spacing: 0.03em;
+}
+
 .main-content {
   flex: 1;
   overflow: hidden;