Camera-based barcode scan page using the native BarcodeDetector API
(no bundler in this project, so no vendored scanning library) with a
manual-entry fallback for unsupported browsers. Adds the
/products/barcode/{barcode} and /api/products/barcode/{barcode}
routes ProductController was missing, plus an "Add to my list" action
on product details. Verified live against the real Open Food Facts
API end to end. "Record price" deferred to CW-EPIC-05, which doesn't
exist yet.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>