Fix review findings: aggregated-refund/saved-card/legacy-refund idempotency keys, structured Square error classification, CSP for Square SDK

Money-safety idempotency fixes (external review bugs 1-3):
- processChargeGroup: aggregated refund key now hashes the sorted pending-row
  set (chargeID-square-agg-<sha256 suffix>) so a changed group can never mark
  a new row completed against an old smaller refund; >45-char chargeIDs use a
  hashed prefix instead of verbatim truncation (which would collide charges on
  Square's global key dedup). Same-set crash-retry keeps Square's dedup.
- CreateTerminalPayment saved_card: two-tier idempotency key — client-supplied
  per-attempt UUID preferred (distinct identical charges no longer collapse),
  deterministic booking+type+amount+card fallback for no-key retry safety.
  PaymentModal sends a per-charge UUID cleared after success.
- ensureRefundKey: legacy NULL-key manual refunds persist a generated key to
  the row BEFORE the Square call (race-safe AND idempotency_key IS NULL guard),
  so a lost-response retry reuses the key and never double-refunds. Wired into
  resumeManualPendingRefund and the sweep's manual-retry loop.

Classification + money-safety hardening:
- till.go/sweep.go: structured square.ErrorCode/IsNotFound are authoritative
  when present; message-substring matching only for non-structured errors
  (dev mock, client-side status errors). Fixes fragile string-matching driving
  sweep retries and gift-card clawbacks.
- SaveCardForUser: ON CONFLICT (user_id, square_card_id) DO NOTHING + re-select
  (was a latent UNIQUE-violation 500 on save-card retry).
- CreateBookingPayment: partial payments re-validated against remaining balance
  inside the advisory lock (closes concurrent-overpayment race).
- InvalidateSquareCustomerCache on GDPR erasure paths (account.go,
  time-blockers.go stale-guest anonymization).
- GetUserGiftCardBalanceAdmin: in-handler admin check (defense-in-depth).
- getCheckoutHTTP: warn on multi-payment checkouts instead of dropping
  payments[1:].
- Cash/giftcard terminal branch: removed dead idempotency SELECT, "tip-" ->
  "till-" prefix.
- UserPaymentModal: removed vestigial polling state; proper interval cleanup.
- account/+page.svelte: gift-card redeem dialog links /terms.
- nginx CSP: allow *.squarecdn.com and js.squareup.com so the Square Web
  Payments SDK + card iframe can tokenize behind the proxy.

Tests: +8 regression tests covering changed-set refund keys, legacy NULL-key
single-refund, saved-card client-key dedup/no-dedup, concurrent partials, and
cache invalidation. Full suite + race detector clean via run-tests.sh lockfile.

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

@@ -35,7 +35,7 @@
 	// the checkbox inside CardSelection; defaults to false (opt-in).
 	let saveCard = $state(false);
 
-	type PaymentStatus = 'idle' | 'processing' | 'polling' | 'success' | 'error';
+	type PaymentStatus = 'idle' | 'processing' | 'success' | 'error';
 
 	let status = $state<PaymentStatus>('idle');
 	let error = $state<string | null>(null);
@@ -102,8 +102,6 @@
 		paymentType = defaultType as 'full' | 'partial' | 'deposit';
 	});
 
-	let pollingInterval: ReturnType<typeof setInterval> | null = null;
-
 	// Payment lock state
 	let lockTimer = $state(-1);
 	let lockAcquired = $state(false);
@@ -506,17 +504,9 @@
 
 	function handleClose() {
 		releaseLock();
-		stopPolling();
 		onClose();
 	}
 
-	function stopPolling() {
-		if (pollingInterval) {
-			clearInterval(pollingInterval);
-			pollingInterval = null;
-		}
-	}
-
 	// Fetch payment methods on mount if authenticated
 	$effect(() => {
 		if (authStore.isAuthenticated) {
@@ -528,7 +518,8 @@
 	// Cleanup on unmount
 	$effect(() => {
 		return () => {
-			stopPolling();
+			if (countdownInterval) clearInterval(countdownInterval);
+			if (lockInterval) clearInterval(lockInterval);
 		};
 	});
 
@@ -927,16 +918,6 @@
 					Close
 				</Button>
 			</div>
-		{:else if status === 'polling'}
-			<!-- Polling State -->
-			<div class="flex flex-col items-center justify-center py-8">
-				<div
-					class="mb-4 h-12 w-12 animate-spin rounded-full border-4 border-gray-200 border-t-green-600"
-				></div>
-				<p class="text-lg font-medium text-gray-700">Processing payment...</p>
-				<p class="mt-2 text-sm text-gray-500">This may take a few moments</p>
-				<Button variant="ghost" onclick={handleClose} class="mt-6">Cancel</Button>
-			</div>
 		{:else if status === 'success' && paymentResult}
 			<!-- Success State -->
 			<div class="space-y-4">