Student profile pages: generate a title and description per student #70

Problem

app/students/[slug]/page.tsx is a client component, so every student profile has the same generic tab title and no description. It's the page a student would share on LinkedIn.

Done when

• Add app/students/[slug]/layout.tsx with generateMetadata({ params }) that looks up the student in data/students.ts and returns title: "<Name> | DevForge", a one-line description, alternates.canonical, and openGraph.images set to their photo.
• An unknown slug still 404s.
• Share-preview check: paste a profile URL into https://www.opengraph.xyz (or view source) and see the right name and photo.
• npm run build passes.

Size: ~25 lines. params is a Promise in this Next version, so await it.

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