feat: implement Supabase authentication with Google OAuth

Add Supabase client, middleware, and auth pages; update header for auth state.

Co-authored-by: ASOwnerYT <[email protected]>

app/auth/callback/route.ts

@@ -0,0 +1,19 @@
+import { createClient } from '@/lib/supabase/server'
+import { NextResponse } from 'next/server'
+
+export async function GET(request: Request) {
+  const { searchParams, origin } = new URL(request.url)
+  const code = searchParams.get('code')
+  const next = searchParams.get('next') ?? '/dashboard'
+
+  if (code) {
+    const supabase = await createClient()
+    const { error } = await supabase.auth.exchangeCodeForSession(code)
+    if (!error) {
+      return NextResponse.redirect(`${origin}${next}`)
+    }
+  }
+
+  // Return the user to an error page with instructions
+  return NextResponse.redirect(`${origin}/auth/error`)
+}