Implement P11: Square Web Payments SDK new-card tokenization

Re-enable new-card entry across all 8 flows via Square Web Payments SDK
cnon: nonces (backend was already P11-ready):
- Add square.ts SDK loader (env-gated on VITE_SQUARE_APPLICATION_ID/LOCATION_ID,
  sandbox vs prod URL auto-derived from app-ID prefix) + SquareCardInput.svelte
  (tokenize() via bind:this, onReady state, CardEntryUnavailable fallback)
- CardSelection.svelte: replace newCardDisabled gate with new-card toggle +
  SquareCardInput; expose tokenize() for parent flows
- Wire new-card mode into tip x3, booking payment (UserPaymentModal), deposit
  (BookingFlow incl. guest), Buy a Gift Card + Add a Card (account), and admin
  till online_square (GiftCardsManagement create/topup)
- Retry-safe: each flow caches the one-shot nonce and reuses it on retry so the
  backend idempotency key dedups instead of re-tokenizing
- Docs: README, Gap Backlog P11, Feature Catalog, Technical Manual, P11 plan

frontend/src/app.d.ts

@@ -14,6 +14,10 @@ declare global {
 		__walkInModalCountdownInterval?: ReturnType<typeof setInterval>;
 		__bookingCreateCountdownInterval?: ReturnType<typeof setInterval>;
 		__bookingFlowCountdownInterval?: ReturnType<typeof setInterval> | null;
+		/** Square Web Payments SDK global, present after square.js loads. */
+		Square?: {
+			payments: (appId: string, locationId: string) => unknown;
+		};
 	}
 }