Unified menu with credits/cash payment toggle

[kylefritz]

Summary

Merge the subscriber and marketplace experiences into a single unified menu where users can choose to pay with credits or cash — similar to Southwest's money/points toggle.

Currently logged-in users can only pay with credits and guests can only pay with cash. This removes that restriction.

Prerequisites

• Rename menu_items.subscriber → menu_items.accepts_credits
• Rename menu_items.marketplace → menu_items.accepts_cash
• Update builder UI labels ("Subscriber"/"Marketplace" → "Accepts Credits"/"Accepts Cash")
• Update admin filters and scopes
• Update Order scopes (.subscriber → .credits, .marketplace → .cash or similar)
• Update subscriber_note → TBD (baker's note? weekly note?)

UI changes

• Single menu view for all users (merge Menu.tsx and Marketplace.tsx)
• Payment toggle (tab or switch) to choose credits vs cash, visible when user has credits
• Items filtered by selected payment method (accepts_credits or accepts_cash)
• Price display adapts: credits when paying with credits, dollars when paying with cash
• Guest users see cash-only view (no toggle since they have no credits)
• Logged-in users with credits see both options

Backend changes

• Order creation accepts explicit payment method choice (not inferred from user context)
• Cart needs to handle both payment types in one flow
• Auth flow: guests may need ability to log in mid-checkout to use credits

Depends on

• Per-mailer email preferences #322 (email preferences) should ship first — independent feature, smaller scope

Open questions

• Can a single order mix credits and cash items?
• What happens to users with 0 credits — do they see the toggle at all?
• Should guests be prompted to log in if they might have credits?