feat(frontend/recs): drop bulk Payment dropdown + add Payment column to table

[cristim]

Problem

Two paired tweaks to the bottom action box + table:

1. Drop the bulk Payment dropdown in
   recommendations.ts:mountBottomActionBox. Each row already carries
   its own payment_option (set by the variant fan-out), and the
   per-cell radio selection enforces one variant per cell — the global
   bulk-payment dropdown is redundant and can mislead the user about
   what's actually being purchased.
2. Add a Payment column to the table. The data exists on every
   rec (payment field) but isn't surfaced in the table — making
   payment-option-by-payment-option comparison hard. The column should
   be sortable and filterable like every other column.

Acceptance criteria

• mountBottomActionBox no longer renders the
  <select id="bulk-purchase-payment"> element.
• All callers reading bulk-purchase-payment.value are updated:
  • The Purchase modal's payment seeding paths in
    resolveBucketPaymentSeed / resolvePerRecPaymentSeed use the
    per-rec payment (already do — verify).
  • loadBulkPurchaseState / saveBulkPurchaseState drop the
    payment field from the persisted toolbar state.
• New "Payment" column in the table between Term and Monthly
  Savings, showing the variant's payment option (all-upfront,
  partial-upfront, no-upfront, monthly).
• Column is in state.RecommendationsColumnId, sortable, and
  filterable (categorical filter — same pattern as Service /
  Account).
• Tests:
  • Bottom action box no longer exposes Payment dropdown.
  • Payment column header renders and sorts.
  • Payment column filter narrows the visible set.