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/whatsapp-attendees/page.tsx

@@ -476,7 +476,7 @@ function WhatsAppAttendeesPageInner() {
       if (templateKey !== "tickets" && !message.trim()) { setError("Message is required"); return; }
       setSending(true);
       const res = await apiFetch(`/api/events/${encodeURIComponent(eventId)}/whatsapp-attendees`, { method: "POST", authToken: token, body: buildPayload() });
-      setInfo(`Sent ${res?.sent ?? 0} out of ${res?.matched ?? 0} recipient(s).`);
+      setInfo(`Queued ${res?.queued ?? res?.matched ?? 0} recipient(s) for sending.`);
       resetAttendeesForm();
     } catch (e: any) { setError(e?.message || "Failed to send"); } finally { setSending(false); }
   };
@@ -655,7 +655,7 @@ function WhatsAppAttendeesPageInner() {
               </div>
               <div className="sm:col-span-2">
                 <div className="text-[11px] text-gray-500 mt-6">
-                  Placeholders: {"{{name}}"}, {"{{event.title}}"}, {"{{event.start}}"}, {"{{balance}}"}
+                  Placeholders: {"{{name}}"}, {"{{event.title}}"}, {"{{event.start}}"}, {"{{balance}}"}, {"{{payment.link}}"}
                 </div>
               </div>
             </div>
@@ -676,6 +676,7 @@ function WhatsAppAttendeesPageInner() {
                         <li><code>{"{{event.title}}"}</code> — the event title.</li>
                         <li><code>{"{{event.start}}"}</code> — the event start date/time.</li>
                         <li><code>{"{{balance}}"}</code> — outstanding balance for the 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="text-[11px] text-gray-500 mt-2">
                         Preference indicators: <span className="text-green-700 bg-green-50 px-1 rounded">WA</span> = WhatsApp only,{" "}