fix(join): persist billingCadence on /join signup

Mirror of the invite-accept fix (c3b1c59) for the self-serve /join path. A joiner who picked annual but abandoned before /api/helcim/subscription ran was left at billingCadence:'monthly' (the model default) with a cadence-unit contributionAmount, rendering $180/mo in admin views.

join.vue now sends cadence to /api/helcim/customer; helcimCustomerSchema accepts cadence (defaults 'monthly'); customer.post.js persists billingCadence in both the new-member create branch and the guest-upgrade $set branch, forced to 'monthly' for $0 signups.

app/pages/join.vue

@@ -432,6 +432,7 @@ const handleSubmit = async () => {
         email: form.email,
         circle: form.circle,
         contributionAmount: form.contributionAmount,
+        cadence: cadence.value,
         agreedToGuidelines: form.agreedToGuidelines,
         billingAddress: form.billingAddress,
       },