Fix early-bird total in notifications; group dashboard registration items

Registration/payment/reminder emails and WhatsApp messages loaded
registrations without their price tranches, so any line spanning more
than one tranche fell back to charging the full quantity at the most
recent tranche's price, silently dropping the early-bird discount from
the outstanding balance and itemized amounts shown to the user.

The user dashboard's registration detail popup also listed one raw
line per tranche; it now merges same item/price/tier lines and groups
early-bird lines separately from standard-price ones.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/src/utils/waMessages.js

@@ -46,8 +46,9 @@ function buildWARegistration(reg, { isNew = true, balance, totalDue, totalPaid }
     ? `You're registered for *${eventTitle}*${eventDate ? ` on ${eventDate}` : ''}.`
     : `Your registration for *${eventTitle}* has been updated.`;
 
+  const { computeOptionLineTotal } = require('./pricing');
   const items = (reg.registrationOptions || [])
-    .map(ro => `- ${ro.eventOption?.name || 'Option'} ×${ro.quantity} — ${fmtAmount((ro.eventOption?.price || 0) * ro.quantity)}`)
+    .map(ro => `- ${ro.eventOption?.name || 'Option'} ×${ro.quantity} — ${fmtAmount(computeOptionLineTotal(ro, null, new Date()))}`)
     .join('\n');
 
   const paid    = totalPaid  ?? (reg.payments || []).reduce((s, p) => s + (p.amount || 0), 0);