Full site redesign, help system, and dashboard stats fixes

Multi-phase visual facelift (design tokens, dashboards, sidebar/navbar
shell, per-page help guides, and a layout/content pass across every
remaining page) plus backend fixes to the dashboard KPI stats:

- Admin/Supervisor dashboard KPIs (revenue, donations, registrations,
  tickets sold) now use a rolling trailing-month window (today back one
  calendar month, e.g. 9 May - 8 June if today is 8 June) instead of
  calendar month-to-date, which under-counted for most of the month.
  The comparison window shifts the same way, so like is still compared
  with like.
- Reports deep-links from those stat tiles now match the same window
  (range=trailing_month, replacing range=this_month).
- Design tokens (brand-* Tailwind scale + shadcn CSS variables), a
  site-wide contextual help button, fixed dashboard sidebar/navbar,
  Admin/Supervisor/Staff/User dashboard rebuilds backed by a new
  GET /api/stats/overview endpoint, a dedicated Contact page, Site
  Settings restyle with WhatsApp config folded in, and an Account
  activity feed backed by a new SecurityEvent model.
- Every remaining page (home, events, registration flow, auth, legal,
  payment results, and every Admin/Supervisor/Staff/User tool page)
  restyled onto the same design tokens, several with real layout
  upgrades (home hero, events list/detail, donate page, auth pages).
- 20+ new dedicated help guides so the whole site has page-specific
  help content instead of falling back to a generic guide.
- Assorted fixes surfaced along the way: donation-leg double-counting
  in payment stats, donations not counting toward revenue, refund
  netting in per-method report breakdowns, and donation
  over-allocation after a refund.

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

frontend/src/components/shared/UserAvatar.tsx

@@ -0,0 +1,23 @@
+import React from "react";
+
+function initialsFrom(name: string): string {
+  const parts = name.trim().split(/\s+/).filter(Boolean);
+  if (parts.length === 0) return "?";
+  return parts
+    .slice(0, 2)
+    .map(p => p[0]!.toUpperCase())
+    .join("");
+}
+
+export function UserAvatar({ name, className = "" }: { name: string; className?: string }) {
+  return (
+    <div
+      className={
+        "shrink-0 rounded-full bg-brand-600 text-white flex items-center justify-center font-semibold " +
+        (className || "w-10 h-10 text-sm")
+      }
+    >
+      {initialsFrom(name)}
+    </div>
+  );
+}