Enable demo mode features, auto-fill demo credentials, and enhance branding integration

- Added `DEMO_MODE` to backend configuration with relaxed security support for specific demo accounts.
- Updated password validators to allow predefined weak passwords in demo mode.
- Auto-fill login forms with demo credentials via query parameters for improved demo accessibility.
- Introduced demo user creation logic during database initialization if `DEMO_MODE` is enabled.
- Replaced `img` tags with `next/image` for consistent and optimized visuals in branding elements.
- Refined footer, header, and layout components to incorporate improved logo handling.

frontend/src/lib/api/hooks/useAuth.ts

@@ -9,7 +9,7 @@
 
 import { useEffect } from 'react';
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
-import { useRouter } from 'next/navigation';
+import { useRouter } from '@/lib/i18n/routing';
 import {
   login,
   register,