feat(signup): unify cadence UX across accept-invite, join, and account

Extract shared SignupFlowOverlay component. Static "Monthly Contribution"
label on all three contribution inputs (was misleadingly dynamic).
"Per Year"/"Per Month" toggle copy; Per Year default on accept-invite,
Per Month default on join. Live billing-summary card on both signup
flows. Welcome-heading on dashboard via ?welcome=1 for new signups.
$0-member polish on account page (hide payment-history + Solidarity
Fund prompts). State-aware contribution-change hint. Invite accept now
creates Helcim customer and sets auth cookie server-side for both free
and paid branches. Pre-registrant invite + /join signup flows manually
verified against Cleo Nguyen preReg and $0-$50 variants.

scripts/reset-invite.cjs

@@ -0,0 +1,34 @@
+require('dotenv').config()
+const mongoose = require('mongoose')
+
+;(async () => {
+  await mongoose.connect(process.env.MONGODB_URI)
+  const db = mongoose.connection.db
+
+  const email = 'jennie+cleonguyen@machinemagic.co'
+
+  const memberRes = await db.collection('members').deleteOne({ email })
+  console.log(`Deleted ${memberRes.deletedCount} member(s)`)
+
+  const preRegRes = await db.collection('preregistrations').updateOne(
+    { email },
+    {
+      $set: { status: 'pending', magicLinkJtiUsed: false },
+      $unset: { acceptedAt: '', memberId: '' },
+    }
+  )
+  console.log(`Reset ${preRegRes.modifiedCount} preRegistration(s)`)
+
+  const member = await db.collection('members').findOne({ email })
+  console.log('\nMember state after reset:')
+  console.log(JSON.stringify(member, null, 2))
+
+  const preReg = await db.collection('preregistrations').findOne(
+    { email },
+    { projection: { email: 1, status: 1, acceptedAt: 1, memberId: 1, magicLinkJtiUsed: 1 } }
+  )
+  console.log('\nPreRegistration state after reset:')
+  console.log(JSON.stringify(preReg, null, 2))
+
+  await mongoose.disconnect()
+})()