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

backend/handlers/payments/giftcards.go

@@ -1,7 +1,6 @@
 package payments
 
 import (
-	"context"
 	"database/sql"
 	"encoding/json"
 	"errors"