fix(events): let guests default to the card rail in the purchase dialog

`selectedMethodId` started as 'lightning', so the "keep the current
selection" guard in the default-rail watcher always found a match and the
guest preference for fiat never applied — verified on demo, where an
anonymous buyer saw "Proceed" (Lightning) preselected with Card on offer.
Start empty and reset to empty on close so each open re-evaluates:
guests land on Card when it exists, everyone else on the first enabled
rail, exactly as the comment already promised.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EYwoAkZZmXMMmaBp4WGUBo

src/modules/events/components/PurchaseTicketDialog.vue

@@ -161,7 +161,9 @@ watch(
 const { providers, providerMeta } = useFiatProviders()
 const { convert } = usePriceConversion()
 
-const selectedMethodId = ref<string>('lightning')
+// Empty until the dialog opens: the watcher below picks the default per
+// rail availability + auth state, so a guest lands on Card, not Lightning.
+const selectedMethodId = ref<string>('')
 const isFiatPending = ref(false)
 const fiatError = ref<string | null>(null)
 
@@ -336,7 +338,7 @@ async function handlePurchase(contact: { name?: string; email: string }) {
 function handleClose() {
   emit('update:isOpen', false)
   resetPaymentState()
-  selectedMethodId.value = 'lightning'
+  selectedMethodId.value = ''
   fiatError.value = null
   isFiatPending.value = false
   quantity.value = 1