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/utils/whatsapp.js

@@ -105,6 +105,12 @@ function isSessionNotFound(e) {
   return msg.includes('session not found') || msg.includes('instance not found');
 }
 
+/** Extracts the WAWP API's own error reason when present, instead of axios's generic
+ *  "Request failed with status code NNN" (which carries no information about what went wrong). */
+function wawpErrorMessage(e) {
+  return e?.response?.data?.message || e?.message || 'Unknown WhatsApp API error';
+}
+
 /**
  * If the WAWP API reports "Session not found", clear the stale instance ID
  * from the DB so the admin UI drops back to the Session Instance setup step.
@@ -221,12 +227,20 @@ async function sendText(toPhone, message) {
   const chatId = toChatId(toPhone);
   if (!chatId) { console.warn('[whatsapp] Invalid phone, skipping text:', toPhone); return; }
   const { token, instanceId } = await getConfig();
-  await axios.post(`${BASE}/send/text`, {
-    access_token: token,
-    instance_id:  instanceId,
-    chatId,
-    message,
-  });
+  try {
+    await axios.post(`${BASE}/send/text`, {
+      access_token: token,
+      instance_id:  instanceId,
+      chatId,
+      message,
+    });
+  } catch (e) {
+    if (isSessionNotFound(e)) {
+      try { await handleSessionNotFound(e); } catch {}
+      throw new Error('Your WhatsApp session has disconnected. Reconnect it in Admin → WhatsApp Settings, then try again.');
+    }
+    throw new Error(`WhatsApp text send failed: ${wawpErrorMessage(e)}`);
+  }
 }
 
 /**
@@ -258,17 +272,26 @@ async function sendPdf(toPhone, localPdfPath, filename, caption) {
   const pdfUrl = `${backendUrl}/uploads/tickets-temp/${tempName}`;
 
   const { token, instanceId } = await getConfig();
-  await axios.post(`${BASE}/send/pdf`, {
-    access_token: token,
-    instance_id:  instanceId,
-    chatId,
-    file: {
-      url:      pdfUrl,
-      filename: filename || 'tickets.pdf',
-      mimetype: 'application/pdf',
-    },
-    caption: caption || '',
-  });
+  try {
+    await axios.post(`${BASE}/send/pdf`, {
+      access_token: token,
+      instance_id:  instanceId,
+      chatId,
+      file: {
+        url:      pdfUrl,
+        filename: filename || 'tickets.pdf',
+        mimetype: 'application/pdf',
+      },
+      caption: caption || '',
+    });
+  } catch (e) {
+    try { fs.unlinkSync(tempPath); } catch {}
+    if (isSessionNotFound(e)) {
+      try { await handleSessionNotFound(e); } catch {}
+      throw new Error('Your WhatsApp session has disconnected. Reconnect it in Admin → WhatsApp Settings, then try again.');
+    }
+    throw new Error(`WhatsApp PDF send failed: ${wawpErrorMessage(e)}`);
+  }
 
   // Clean up after 5 minutes — WAWP will have fetched the file by then
   setTimeout(() => { try { fs.unlinkSync(tempPath); } catch {} }, 5 * 60 * 1000);