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>

backend/src/utils/notifications.js

@@ -23,7 +23,7 @@ const { getSettingSync } = require('./settingsCache');
 
 function getOrg() {
   return {
-    name:  getSettingSync('org_name', process.env.ORG_NAME || 'Hope Events'),
+    name:  getSettingSync('org_name', process.env.ORG_NAME || 'Cross Code'),
     email: process.env.EMAIL_FROM || process.env.EMAIL_USER || '',
     url:   (process.env.FRONTEND_URL || process.env.APP_BASE_URL || 'http://localhost:3001').replace(/\/$/, ''),
   };