Fix review findings: expiry bug (all 8 files), idempotency keys, card_expiry/card_cvc removal, URL encoding, BuyerEmail logging, ValidateCardInfo, saved-card test, future work doc

Backend:
- Fix refund idempotency key: clock.Now() → deterministic (pr.ID + amount)
- Fix ValidateCardInfo: enforce mutual exclusivity, handle empty strings symmetrically
- Fix paymentFromSquare brand fallback (remove dead SourceType fallback)
- Fix URL encoding: PathEscape → QueryEscape for customer_id query param
- Fix BuyerEmail: log warning on DB error instead of silent discard
- Fix idempotency key in createCardOnFileHTTP: time.Now() → deterministic hex hash
- Add BuyerEmail to CreateTipPayment Square request
- Move realBaseURL from shared file to square_dev.go (only used in dev)
- Add TestTipPayment_WithSavedCard test (card_id path coverage)
- Fix AMEX brand in mock (AMEX → AMERICAN_EXPRESS, fix test)

Frontend:
- Fix off-by-month expiry bug in ALL 8 files using year-month arithmetic
  (parseExpiryParts returns 1-indexed, SvelteDate expects 0-indexed)
  Files: tip/+page, pay-tip/[id], UserBookingModal, UserPaymentModal,
  BookingFlow, account/+page (add card + buy gift card sections)
- Remove card_expiry/card_cvc from tip request bodies (backend has no fields)

Docs:
- Mark P9 (placeholder tokens) as completed, add P11 (Square Web Payments SDK)
- Mark T13 (rune arithmetic) as completed

frontend/src/lib/components/booking/BookingFlow.svelte

@@ -136,7 +136,7 @@
 	const cardError = $derived(
 		cardNumberTouched && !isValidLuhn(newCardNumber) && newCardNumber.length > 0
 			? 'Invalid card number'
-			: cardExpiryTouched && expiryParts !== null && (() => { const d = new SvelteDate(expiryParts.year, expiryParts.month); return d < new SvelteDate(); })()
+			: cardExpiryTouched && expiryParts !== null && (() => { const em = expiryParts.year * 12 + expiryParts.month; const now2 = new SvelteDate(); const cm = now2.getFullYear() * 12 + now2.getMonth() + 1; return em < cm; })()
 				? 'This card has expired'
 				: cardExpiryTouched && !/^\d{2}\/\d{2}$/.test(newCardExpiry) && newCardExpiry.length > 0
 					? 'Enter expiry as MM/YY'