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/scripts/migrate-env-to-db.js

@@ -64,8 +64,9 @@ const MAPPINGS = [
   { key: 'org_tagline',            envVars: ['ORG_TAGLINE']                                       },
   { key: 'org_email',              envVars: ['EMAIL_FROM', 'EMAIL_USER', 'SMTP_FROM', 'SMTP_USER'] },
 
-  // Branding
-  { key: 'accent_color',           envVars: ['EMAIL_HEADER_COLOR', 'BRAND_COLOR']                 },
+  // Branding — these env vars historically fed the single "brand color" concept,
+  // which is now the Primary color in the 3-color Primary/Secondary/Accent system.
+  { key: 'primary_color',          envVars: ['EMAIL_HEADER_COLOR', 'BRAND_COLOR']                 },
 
   // Notifications
   { key: 'reg_notification_emails',envVars: ['REGISTRATIONS_EMAIL']                               },
@@ -88,7 +89,7 @@ async function main() {
   const prisma = new PrismaClient();
 
   console.log(`\n${'─'.repeat(60)}`);
-  console.log('  Hope Events — .env → DB settings migration');
+  console.log('  Settings migration: .env → DB');
   if (DRY_RUN) console.log('  MODE: DRY RUN (no changes will be made)');
   if (FORCE)   console.log('  MODE: FORCE (will overwrite existing DB values)');
   console.log(`${'─'.repeat(60)}\n`);