Square payment integration: real HTTP client, tip flow rewrite, card UI/validation overhaul

Backend:
- Create square_http_client.go: real Square REST API client (Payments, Terminal Checkouts, Refunds, Cards, Locations) with proper JSON types, auth, error handling
- Update ProdClient in square.go to delegate to shared HTTP functions
- Wire devProdClient in square_dev.go to also make real HTTP calls for sandbox/prod env
- Rewrite CreateTipPayment handler: accept card_id OR new_card_token (+save_card), advisory lock, idempotency check, max amount validation
- Add ValidateCardInfo, bump ValidateAmount max to £10,000
- Fix mock CreateCardOnFile to detect brand/last4 from raw card numbers
- Fix mock RefundPayment to index by SquarePayID and accept unknown payment IDs
- Remove dead types (ProcessingFee, sqAddress), add Deadline parity
- Fix AMEX brand inconsistency (AMEX -> AMERICAN_EXPRESS)
- Pre-existing fix: remove unused context import in giftcards.go

Frontend:
- CardInput.svelte: add onfieldblur/onfieldinput callbacks for blur-based validation
- CardBrandIcon.svelte: brand SVGs for VISA, MC, AMEX, Discover, Diners, JCB, Square Gift Card, UnionPay, Interac, EFTPOS
- tip/+page, pay-tip/[id], UserBookingModal tip: saved card list + CardInput + Luhn/expiry/CVC validation + blur-based errors + no-saved-cards edge case
- UserPaymentModal, BookingFlow: card validation parity (blur-based, all-valid check)
- account page: replace text brand badges with CardBrandIcon
- Fix handleCustomTip bug (state mutations outside if block)
- Remove dead pageState variable
- Add tip modal scroll (max-h-[90vh] overflow-y-auto)
- Submit button disabled on !isCardValid

Tests:
- 30 square package tests (+new: CreateCardOnFile raw number path, detectCardInfo variants)
- 5 tip handler tests (HappyPath, NoPriorPayment, WrongOwner, MultipleTips, TxFailure)
- All +-race clean, refund tests fixed

frontend/src/lib/components/payments/UserPaymentModal.svelte

@@ -61,6 +61,9 @@
 	let newCardExpiry = $state('');
 	let newCardCVC = $state('');
 	let saveCardForFuture = $state(false);
+	let cardNumberTouched = $state(false);
+	let cardExpiryTouched = $state(false);
+	let cardCVCTouched = $state(false);
 
 	function parseExpiryParts(value: string): { month: number; year: number } | null {
 		if (!/^\d{2}\/\d{2}$/.test(value)) return null;
@@ -99,6 +102,18 @@
 		return sum % 10 === 0 && s.length >= 13 && s.length <= 19;
 	}
 
+	function handleFieldBlur(field: string) {
+		if (field === 'cardNumber') cardNumberTouched = true;
+		else if (field === 'cardExpiry') cardExpiryTouched = true;
+		else if (field === 'cardCVC') cardCVCTouched = true;
+	}
+
+	function handleFieldInput(field: string) {
+		if (field === 'cardNumber') cardNumberTouched = false;
+		else if (field === 'cardExpiry') cardExpiryTouched = false;
+		else if (field === 'cardCVC') cardCVCTouched = false;
+	}
+
 	const cardFormValid = $derived(
 		isValidLuhn(newCardNumber) && expiryParts !== null && newCardCVC.length >= 3 && !isExpiryInPast
 	);
@@ -113,19 +128,21 @@
 		!cardSelected
 			? selectedPaymentMethod === null && paymentMethods.length > 0 && !showNewCardForm
 				? 'Please select a card'
-				: !isValidLuhn(newCardNumber) && newCardNumber.length > 0
+				: cardNumberTouched && !isValidLuhn(newCardNumber) && newCardNumber.length > 0
 					? 'Invalid card number'
-					: hasInvalidMonth
+					: cardExpiryTouched && hasInvalidMonth
 						? 'Invalid expiry month'
-						: isExpiryInPast
+						: cardExpiryTouched && isExpiryInPast
 							? 'Expiry date in the past'
-							: !/^\d{2}\/\d{2}$/.test(newCardExpiry) && newCardExpiry.length > 0
+							: cardExpiryTouched && !/^\d{2}\/\d{2}$/.test(newCardExpiry) && newCardExpiry.length > 0
 								? 'Enter expiry as MM/YY'
-								: newCardCVC.length < 3 && newCardCVC.length > 0
+								: cardCVCTouched && newCardCVC.length < 3 && newCardCVC.length > 0
 									? 'Enter your CVC number'
-									: paymentMethods.length === 0 && !showNewCardForm && newCardNumber.length === 0
-										? 'Please enter card details'
-										: 'Please complete all card fields'
+									: isValidLuhn(newCardNumber) && /^\d{2}\/\d{2}$/.test(newCardExpiry) && newCardCVC.length >= 3
+										? null
+										: newCardNumber.length === 0 && newCardExpiry.length === 0 && newCardCVC.length === 0
+											? null
+											: 'Please complete all card fields'
 			: null
 	);
 
@@ -842,6 +859,8 @@
 							bind:saveCard={saveCardForFuture}
 							showSaveCard={canSaveCards}
 							disabled={false}
+							onfieldblur={handleFieldBlur}
+							onfieldinput={handleFieldInput}
 						/>
 					{/if}