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/docs/branding/02-visual-identity.md

@@ -12,6 +12,7 @@ The **PragmaStack** logo represents the core values of the project: structure, s
 </div>
 
 ### Icon
+
 For smaller contexts (favicons, headers), we use the simplified icon:
 
 <div align="center">