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>

frontend/src/app/dashboard/supervisor/email-attendees/page.tsx

@@ -466,8 +466,8 @@ function EmailAttendeesPageInner() {
         if (body.trim().startsWith('<')) payload.html = body; else payload.text = body.replace(/\n/g, '\n');
       }
       const res = await apiFetch(`/api/events/${encodeURIComponent(eventId)}/email-attendees`, { method: 'POST', authToken: token, body: payload });
-      const sent = res?.sent ?? 0; const matched = res?.matched ?? 0;
-      setInfo(`Sent ${sent} out of ${matched} recipient(s).`);
+      const queued = res?.queued ?? res?.matched ?? 0;
+      setInfo(`Queued ${queued} recipient(s) for sending.`);
       // Reset form to default state
       resetAttendeesForm();
     } catch (e: any) {
@@ -552,7 +552,7 @@ function EmailAttendeesPageInner() {
                 </select>
               </div>
               <div className="sm:col-span-2">
-                <div className="text-[11px] text-gray-500 mt-6">Available placeholders: {'{{name}}'}, {'{{event.title}}'}, {'{{event.start}}'}, {'{{balance}}'} <button type="button" className="ml-2 underline hover:no-underline" onClick={() => setShowInfo(true)}>Learn more</button></div>
+                <div className="text-[11px] text-gray-500 mt-6">Available placeholders: {'{{name}}'}, {'{{event.title}}'}, {'{{event.start}}'}, {'{{balance}}'}, {'{{payment.link}}'} <button type="button" className="ml-2 underline hover:no-underline" onClick={() => setShowInfo(true)}>Learn more</button></div>
               </div>
             </div>
 
@@ -572,9 +572,10 @@ function EmailAttendeesPageInner() {
                         <li><code>{'{{event.title}}'}</code> — the event title.</li>
                         <li><code>{'{{event.start}}'}</code> — the event start date/time (local).</li>
                         <li><code>{'{{balance}}'}</code> — outstanding amount across the attendee’s registrations for the selected event.</li>
+                        <li><code>{'{{payment.link}}'}</code> — a direct Yoco payment link for the attendee's outstanding balance (generated per recipient when sending).</li>
                       </ul>
-                      <p className="mb-2">Example: Hi <code>{'{{name}}'}</code>, your balance is <code>{'{{balance}}'}</code>.</p>
-                      <p className="text-[11px] text-gray-500">To add new placeholders, extend the replacement logic in <span className="font-mono">eventController.emailEventAttendees</span> (replacePlaceholders function) and update this help.</p>
+                      <p className="mb-2">Example: Hi <code>{'{{name}}'}</code>, your balance is <code>{'{{balance}}'}</code>. Pay here: <code>{'{{payment.link}}'}</code></p>
+                      <p className="text-[11px] text-gray-500">To add new placeholders, extend <span className="font-mono">backend/src/utils/placeholders.js</span> and update this help.</p>
                     </div>
                   </div>
                 </div>