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/app/layout.tsx

@@ -6,6 +6,7 @@ import { SiteSettingsProvider } from "@/contexts/SiteSettingsContext";
 import { ToastProvider } from "@/components/shared/ToastProvider";
 import { BannerBar } from "@/components/shared/BannerBar";
 import { SetupGuard } from "@/components/shared/SetupGuard";
+import HelpFab from "@/components/shared/HelpFab";
 
 const geistSans = Geist({
   variable: "--font-geist-sans",
@@ -37,6 +38,7 @@ export default function RootLayout({ children }: { children: React.ReactNode })
                             <SetupGuard />
                             <BannerBar />
                             {children}
+                            <HelpFab />
                         </ToastProvider>
                     </AuthProvider>
                 </SiteSettingsProvider>