Add a page title and description to the QR generator (/qr) #63

Problem

app/qr/page.tsx has no metadata of its own.

Why

app/qr/page.tsx is a client component ("use client"), so it can't export metadata. The browser tab and search results fall back to the site-wide title, so it's indistinguishable from the home page in a row of tabs or in Google.

Done when

• Add app/qr/layout.tsx (a server component that just returns children) exporting metadata with a specific title and a one-sentence description. Follow the pattern in app/learn/open-source/page.tsx.
• Also set alternates: { canonical: "/qr" }.
• The tab title is correct on /qr, and npm run build passes.

Size: ~15 lines.

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