feat: complete Week 1 frontend setup (#9 #10 #11 #12)

- Install and configure Tailwind CSS (#9)
- Install Nuxt UI component library (#10)
- Create app layout with header/footer components (#11)
- Implement Supabase client composable (#12)
- Add TypeScript database types
- Create placeholder pages (index, scan, settings)
- Setup responsive navigation with mobile menu
- Configure auth state management

All Week 1 frontend foundation tasks complete.

app/composables/useSupabase.ts

@@ -0,0 +1,81 @@
+import { createClient, SupabaseClient } from '@supabase/supabase-js'
+import type { Database } from '~/types/database.types'
+
+let supabaseInstance: SupabaseClient<Database> | null = null
+
+export const useSupabase = () => {
+  const config = useRuntimeConfig()
+  
+  if (!supabaseInstance) {
+    supabaseInstance = createClient<Database>(
+      config.public.supabaseUrl,
+      config.public.supabaseAnonKey,
+      {
+        auth: {
+          persistSession: true,
+          autoRefreshToken: true,
+          detectSessionInUrl: true,
+          storage: process.client ? window.localStorage : undefined
+        }
+      }
+    )
+  }
+
+  return supabaseInstance
+}
+
+/**
+ * Composable for Supabase authentication
+ */
+export const useSupabaseAuth = () => {
+  const supabase = useSupabase()
+  const user = useState('supabase_user', () => null as any)
+  const session = useState('supabase_session', () => null as any)
+
+  // Initialize auth state
+  const initAuth = async () => {
+    const { data: { session: currentSession } } = await supabase.auth.getSession()
+    session.value = currentSession
+    user.value = currentSession?.user || null
+
+    // Listen for auth changes
+    supabase.auth.onAuthStateChange((_event, newSession) => {
+      session.value = newSession
+      user.value = newSession?.user || null
+    })
+  }
+
+  // Call initAuth on composable mount
+  if (process.client) {
+    initAuth()
+  }
+
+  const signIn = async (email: string, password: string) => {
+    const { data, error } = await supabase.auth.signInWithPassword({
+      email,
+      password
+    })
+    return { data, error }
+  }
+
+  const signUp = async (email: string, password: string) => {
+    const { data, error } = await supabase.auth.signUp({
+      email,
+      password
+    })
+    return { data, error }
+  }
+
+  const signOut = async () => {
+    const { error } = await supabase.auth.signOut()
+    return { error }
+  }
+
+  return {
+    user: readonly(user),
+    session: readonly(session),
+    signIn,
+    signUp,
+    signOut
+  }
+}