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/lib/theme.ts

@@ -0,0 +1,20 @@
+/**
+ * Hex mirrors of the `brand` scale in tailwind.config.js, for the rare spot
+ * that needs a literal color value instead of a Tailwind class (SVG
+ * stroke/fill, canvas, etc.). Keep these in sync with tailwind.config.js by
+ * hand — there are few enough call sites that a build-time sync step isn't
+ * worth it.
+ *
+ * Do NOT use these for the org name/heading text — that stays bound to
+ * SiteSettingsContext.settings.accent_color, not the fixed brand palette.
+ */
+export const BRAND_50 = "#EEF2FF";
+export const BRAND_100 = "#E0E7FF";
+export const BRAND_200 = "#C7D2FE";
+export const BRAND_300 = "#A5B4FC";
+export const BRAND_400 = "#818CF8";
+export const BRAND_500 = "#6366F1";
+export const BRAND_600 = "#4F46E5";
+export const BRAND_700 = "#4338CA";
+export const BRAND_800 = "#3730A3";
+export const BRAND_900 = "#312E81";