Improve tooltip styles and guard search query

Add Vuetify tooltip CSS for better contrast and readability in light/dark themes (sets background/text colors, font-size, weight, padding, radius, shadow and forces opacity) in GUI/src/global.css. In AdminUsers.vue, guard against null/undefined `searchQuery.value` by using `searchQuery.value ?? ''` before trimming and lowercasing to prevent runtime errors when the query is unset.

GUI/src/global.css

@@ -784,3 +784,15 @@ pre {
     font-size: var(--font-size-xs);
   }
 }
+
+/* Vuetify Tooltip – lesbarer Kontrast in Light & Dark */
+.v-tooltip > .v-overlay__content {
+  background-color: var(--color-text) !important;
+  color: var(--color-bg) !important;
+  font-size: var(--font-size-xs);
+  font-weight: 500;
+  padding: 6px 10px;
+  border-radius: var(--radius-sm, 6px);
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.25);
+  opacity: 1 !important;
+}

GUI/src/routes/admin/AdminUsers.vue

@@ -26,7 +26,7 @@ const adminCount = computed(
 )
 
 const filteredUsers = computed(() => {
-  const query = searchQuery.value.trim().toLowerCase()
+  const query = (searchQuery.value ?? '').trim().toLowerCase()
   if (query.length === 0) {
     return users.value
   }