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/mint-invite-link.cjs

@@ -0,0 +1,48 @@
+require('dotenv').config()
+const mongoose = require('mongoose')
+const jwt = require('jsonwebtoken')
+const { randomUUID } = require('crypto')
+
+const BASE_URL = process.argv[2]
+const EMAIL = process.argv[3] || 'jennie+cleonguyen@machinemagic.co'
+
+if (!BASE_URL) {
+  console.error('Usage: node scripts/mint-invite-link.cjs <base-url> [email]')
+  process.exit(1)
+}
+
+const secret = process.env.NUXT_JWT_SECRET || process.env.JWT_SECRET
+if (!secret) {
+  console.error('Missing NUXT_JWT_SECRET / JWT_SECRET in .env')
+  process.exit(1)
+}
+
+;(async () => {
+  await mongoose.connect(process.env.MONGODB_URI)
+  const db = mongoose.connection.db
+
+  const preReg = await db.collection('preregistrations').findOne({ email: EMAIL })
+  if (!preReg) {
+    console.error(`No preregistration found for ${EMAIL}`)
+    await mongoose.disconnect()
+    process.exit(1)
+  }
+
+  const jti = randomUUID()
+  const token = jwt.sign(
+    { preRegistrationId: preReg._id.toString(), jti, type: 'prereg-invite' },
+    secret,
+    { expiresIn: '48h' },
+  )
+
+  await db.collection('preregistrations').updateOne(
+    { _id: preReg._id },
+    { $set: { magicLinkJti: jti, magicLinkJtiUsed: false, status: 'invited' } },
+  )
+
+  const link = `${BASE_URL.replace(/\/$/, '')}/accept-invite#${token}`
+  console.log('\nFresh invite link for', EMAIL, ':\n')
+  console.log(link, '\n')
+
+  await mongoose.disconnect()
+})()