Act as a senior web developer and marketing expert. I want to build a website for my marketing agency. We offer web development, SEO, paid ads, and lead generation for local small businesses. Use Next.js 15 with the App Router and Tailwind CSS. Look at https://www.webfx.com/ for the style I want. The site must be fully responsive for mobile users. First, explain your thinking for the site structure. Second, create a step-by-step build plan, including how to handle form errors or 404 pages. Third, ask me 5 clarifying questions about the design and content before we start.