feat(checkout): read sol_price_usd from payment object (closes #1 widget portion)

[relayhop]

Closes Cypher-CP0/fluxpay#1 (widget portion). Pairs with Cypher-CP0/fluxpay#2 (backend portion).

Problem

The widget made a second HTTP call to GET /api/price/sol to learn the SOL/USD rate, even though the issue says: Read sol_price_usd from the payment object returned by the backend.

Change

• src/types.ts: add optional sol_price_usd?: number to the Payment interface (optional preserves backward compatibility with backends that have not been upgraded yet).
• src/CheckoutModal.tsx: prefer payment.sol_price_usd once the polled payment object provides it; fall back to a single /api/price/sol GET if the backend has not been upgraded.
• Initial solPrice state is null instead of the hardcoded 165, so the widget waits for a real price before showing a SOL amount (renders 0 SOL for the brief instant before the first poll lands rather than a misleading number).

Result

• One fewer HTTP roundtrip per checkout once the backend returns sol_price_usd.
• The displayed SOL amount matches what the backend computed at payment-creation time (no oracle drift between the widget price call and the create call).
• Older backends still work — the fallback path is preserved.

Tests

Manual:

• New backend (with fix: wallet connection unreliable on first select —> WalletNotSelectedError #2 merged) → widget reads payment.sol_price_usd, no /api/price/sol request in network tab.
• Old backend → widget falls back to /api/price/sol exactly once.