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/wizard/SetupWizard.vue

@@ -8,7 +8,7 @@ import ThemeToggle from '@/components/layout/ThemeToggle.vue'
 
 const router = useRouter()
 const app = useAppStore()
-const theme = useThemeStore()
+useThemeStore() // ensures theme is initialized before ThemeToggle renders
 
 const step = ref(1)
 const totalSteps = 3