fix: remove unused imports/variables caught by vue-tsc

- Remove unused app import in Sidebar.vue
- Remove unused toggleMode function in ThemeToggle.vue
- Remove unused theme variable in SetupWizard.vue
- Remove unused watch import in stores/theme.ts

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

frontend/src/components/layout/Sidebar.vue

@@ -2,15 +2,12 @@
 import { ref, onMounted } from 'vue'
 import { useRouter, useRoute } from 'vue-router'
 import { gql } from '@/lib/gql'
-import { useAppStore } from '@/stores/app'
-
 const emit = defineEmits<{ close: [] }>()
 
 interface DocMeta { slug: string; title: string }
 
 const docs = ref<DocMeta[]>([])
 const loading = ref(true)
-const app = useAppStore()
 const router = useRouter()
 const route = useRoute()