docs: add backend integration guide + implementation prompt for B2B
Document how the B2B storefront sends/gets data vs main (base-URL
resolution, bootstrap fetch, interceptor chain, headers) and the new
builder/backoffice surface awaiting a real API. Add a self-contained
hand-off prompt. Login and payments left untouched.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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