fix(frontend): use real merchant API data in ExploreMap offer cards (#151)

[gidadoabdullateef5]

Summary

Fixes #151 — ExploreMap was showing identical placeholder economics on every merchant card because merchantToOffer ignored the fields returned by GET /merchants/available.

This PR wires merchantToOffer to the existing AvailableMerchant response so each card reflects that merchant's real distance, payout, and commission.

What changed

File: micopay/frontend/src/pages/ExploreMap.tsx

merchantToOffer now maps API fields instead of hardcoded constants:

API field (AvailableMerchant)	UI field (Offer)	Helper used
distance_km	distance	formatDistance()
distance_km	walkMinutes	walkMinutes()
payout_mxn	receiveMxn	—
rate_percent	commissionPct	—
username	name	(already wired)

Before: every card showed 180 m, 3 min, $495.00 MXN, and 1% commission.

After: each card shows values computed from that merchant's API response.

Also removed the hardcoded badge: 'Negocio verificado' since AvailableMerchant has no badge field — the badge UI remains optional and simply won't render when absent.

Acceptance criteria

• merchantToOffer maps m.distance_km → formatDistance / walkMinutes
• merchantToOffer maps m.payout_mxn → receiveMxn and m.rate_percent → commissionPct
• Each merchant card displays that merchant's real distance, payout, and commission
• Empty merchant list handled gracefully via existing EmptyState (no crash)

Scope

Frontend only — single-file change in ExploreMap.tsx. No backend or hook changes required; useMerchantsAvailable already returns the correct data.

Test plan

• Open Convertir a efectivo with location permission granted
• Confirm multiple merchant cards show different distances, walk times, payouts, and commission percentages
• Confirm the primary ("Mejor oferta") card uses real values from the first merchant in the API response
• Confirm commission display ($X.XX (Y%)) matches amount - payout_mxn and rate_percent
• Test with no nearby merchants → empty state renders without errors
• Test location denied / fetch error paths still work (unchanged)

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