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.

.env.template

@@ -17,6 +17,7 @@ BACKEND_PORT=8000
 # Must be at least 32 characters
 SECRET_KEY=your_secret_key_here_REPLACE_WITH_GENERATED_KEY_32_CHARS_MIN
 ENVIRONMENT=development
+DEMO_MODE=false
 DEBUG=true
 BACKEND_CORS_ORIGINS=["http://localhost:3000"]
 FIRST_SUPERUSER_EMAIL=admin@example.com