Add admin-configurable branding (colors, logo, favicon) and generic default fallbacks

Site Settings -> Branding now supports a Primary/Secondary/Accent brand color
system applied site-wide (buttons, nav, hover states, links) and to outgoing
email header/CTA colors, plus a favicon upload alongside the existing logo
upload, a live preview panel (website/email x desktop/mobile), and
logo-based color suggestions. The setup wizard's Branding step got the same
treatment. Fixes two related bugs found along the way: the setup wizard's
logo/favicon upload was missing its auth token, and a static favicon.ico in
Next's special app/ convention path was silently overriding the dynamic one.

Also replaces every "Hope Events"/"Hope Family Church" default (org name,
email subjects, WhatsApp messages, report metadata, API docs) with a neutral
"Cross Code" placeholder, and the optional legal settings (operator name, IO
details, website URL, effective date) with obviously-generic placeholders
instead of defaulting to real personal/organisational details -- since this
platform is deployed for multiple organisations. Adds SETTINGS.md documenting
every setting's default behaviour.

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

frontend/tailwind.config.js

@@ -39,22 +39,24 @@ module.exports = {
 					DEFAULT: "hsl(var(--card))",
 					foreground: "hsl(var(--card-foreground))",
 				},
-				// Fixed indigo/purple brand scale — the site's hard-coded design-system
-				// color. Use this (not `indigo-*`/`blue-*` literals) for anything that
-				// isn't a shadcn primitive: nav active-states, icon chips, the help FAB,
-				// etc. Never bind this scale to the admin-configurable accent_color
-				// setting (see globals.css) — that stays an inline style on the org name only.
+				// Brand scale — the site's primary design-system color. Use this (not
+				// `indigo-*`/`blue-*` literals) for anything that isn't a shadcn
+				// primitive: nav active-states, icon chips, the help FAB, etc.
+				// Values come from CSS custom properties set at request-time in
+				// layout.tsx from the admin's saved `primary_color` setting (see
+				// colorScale.ts's buildBrandScale + globals.css for the static
+				// fallback defaults used when no setting is saved yet).
 				brand: {
-					50: "#EEF2FF",
-					100: "#E0E7FF",
-					200: "#C7D2FE",
-					300: "#A5B4FC",
-					400: "#818CF8",
-					500: "#6366F1",
-					600: "#4F46E5",
-					700: "#4338CA",
-					800: "#3730A3",
-					900: "#312E81",
+					50: "var(--brand-50)",
+					100: "var(--brand-100)",
+					200: "var(--brand-200)",
+					300: "var(--brand-300)",
+					400: "var(--brand-400)",
+					500: "var(--brand-500)",
+					600: "var(--brand-600)",
+					700: "var(--brand-700)",
+					800: "var(--brand-800)",
+					900: "var(--brand-900)",
 				},
 			},
 			borderRadius: {