Make theme toggle-able.

services/frontend/src/components/NavigationBar.vue

+<script setup>
+  import { useTheme } from 'vuetify'
+
+  const theme = useTheme()
+
+  function toggleTheme () {
+    theme.global.name.value = theme.global.current.value.dark ? 'light' : 'dark'
+}
+</script>
+
 <template>
   <v-app-bar
     app
@@ -48,9 +58,9 @@
     >
       git
     </v-btn>
+    <v-btn @click="toggleTheme">
+      toggle theme
+    </v-btn>
     <v-spacer />
   </v-app-bar>
 </template>
-
-<script setup>
-</script>