bug(ui/purchases): multi-bucket Configure Purchase modal seeds 'all-upfront' regardless of per-rec payment #699

When fan-out into multiple buckets, Configure Purchase modal sets PaymentOption='all-upfront' for ALL selected recommendations regardless of what each row's actual payment option is. Single-bucket flow (openPurchaseModal → resolvePerRecPaymentSeed) handles per-rec payment correctly; the multi-bucket fan-out (resolveBucketPaymentSeed in frontend/src/recommendations.ts:3242-3305) seeds from toolbar.payment (=cachedGlobalDefaultPayment, default all-upfront) and never inspects rec.payment. Silently mis-bills users on bulk purchases.

Fix direction

Include payment in the bucket key at recommendations.ts:3156 so recs with different payments fan into separate buckets (each bucket then uniform on payment, seed becomes recs[0].payment). Mirrors the existing 'bundle B' term-key approach. Alternative: extend resolveBucketPaymentSeed to fall back to recs[0].payment when all recs in the bucket share one supported value before falling back to toolbar — less clean.

Surfaced by QA spreadsheet row 5.2.