Fix financial double-counting, rebuild cashup accountability, and redesign the Reports page

Financial correctness (donation-leg model):
- Donations are no longer mutated when assigned to a registration; assignment now
  creates an immutable "leg" record referencing the original donation instead.
- Fixed several places where money was double-counted once a donation was partially
  or fully assigned (Payments, Revenue summary, Cashup reconciliation, Finance
  report, Profit report, Master Orders, Revenue Detailed).
- Payments now record who recorded them (recordedBy), separate from who they're for.

Cashup:
- Per-user cash denomination counting (optional, any time) replaces the single
  event-wide manual entry; the event's cash actual is the live sum of these counts.
- New "Payment accountability by staff member" breakdown across all methods, and a
  read-only "Report" tab that opens automatically once an event is closed.

Reports page redesign:
- New shell: sidebar of universal filters (events, date range, past/inactive/closed
  toggles), searchable/categorized report grid, and a popup viewer with
  Print/Email/Excel/WhatsApp actions plus an in-app Reporting Guide.
- Visual pass: colored stat tiles and bar charts on most reports, matching mockups.
- PDF exports (download/Print/Email/WhatsApp) now share a branded design mirroring
  the web report — colored header, stat tiles, bar chart, highlighted totals.
- Excel export now produces a styled .xlsx (via exceljs) instead of a plain CSV.
- Master Orders' "Donations made" table is now included in every export channel.

Bug fixes discovered while testing exports:
- Report emails now go through the shared, DB-configurable mail utility instead of
  a one-off transporter that ignored Site Settings SMTP config.
- WhatsApp report sends now surface the actual WAWP API error and auto-recover a
  disconnected session, instead of a bare axios status-code message.

Also: Admin-editable notification preference, richer Admin Registrations dashboard,
{{payment.link}} placeholder for Email/WhatsApp Attendees, and background
email/WhatsApp attendee sending.

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

backend/src/controllers/userController.js

@@ -354,21 +354,15 @@ const updateUserProfile = async (req, res) => {
     }
 
     // Normalize phone
-    const { normalizeZAPhone, isValidZAPhone } = require('../utils/whatsapp');
+    const { normalizeZAPhone } = require('../utils/whatsapp');
     let newPhone = user.phoneNumber;
     if (phoneNumber !== undefined) {
       newPhone = phoneNumber ? (normalizeZAPhone(phoneNumber) || phoneNumber.replace(/\D/g, '') || null) : null;
     }
 
     // Validate notification preference — WhatsApp requires a valid SA phone number
-    const allowedPrefs = ['email', 'whatsapp', 'both'];
-    let newPref = user.notificationPreference;
-    if (notificationPreference !== undefined) {
-      newPref = allowedPrefs.includes(notificationPreference) ? notificationPreference : user.notificationPreference;
-      if ((newPref === 'whatsapp' || newPref === 'both') && !isValidZAPhone(newPhone)) {
-        newPref = 'email';
-      }
-    }
+    const { resolveNotificationPreference } = require('../utils/notificationPreference');
+    const newPref = resolveNotificationPreference(notificationPreference, newPhone, user.notificationPreference);
 
     // Update user data
     const updatedUser = await prisma.user.update({
@@ -554,7 +548,11 @@ const updateUser = async (req, res) => {
       throw new Error('User not found');
     }
 
-    const { name, email, role, isActive, phoneNumber, password } = req.body;
+    const { name, email, role, isActive, phoneNumber, password, notificationPreference } = req.body;
+
+    const newPhone = phoneNumber !== undefined ? (phoneNumber || null) : user.phoneNumber;
+    const { resolveNotificationPreference } = require('../utils/notificationPreference');
+    const newPref = resolveNotificationPreference(notificationPreference, newPhone, user.notificationPreference);
 
     // Prepare data update, allow admin to set a new password
     const data = {
@@ -562,7 +560,8 @@ const updateUser = async (req, res) => {
       email: email || user.email,
       role: role || user.role,
       isActive: isActive !== undefined ? isActive : user.isActive,
-      phoneNumber: phoneNumber !== undefined ? (phoneNumber || null) : user.phoneNumber,
+      phoneNumber: newPhone,
+      notificationPreference: newPref,
       updatedAt: new Date()
     };
 
@@ -579,6 +578,7 @@ const updateUser = async (req, res) => {
         email: true,
         role: true,
         phoneNumber: true,
+        notificationPreference: true,
         createdAt: true,
         updatedAt: true,
         isActive: true