Add dynamic welcome message configuration

[heysamtexas]

Description

Allow admins to configure custom welcome messages for new users through the Django admin interface.

Current State

Welcome messages are likely hardcoded in templates or not present.

Proposed Implementation

Add fields to SiteConfiguration model:

• welcome_message - TextField for main welcome text
• welcome_message_enabled - BooleanField to toggle display

Use Cases

• Seasonal messaging
• Feature announcements
• Onboarding guidance
• A/B testing different messages

Acceptance Criteria

• Add welcome message fields to SiteConfiguration
• Display message prominently for new/first-time users
• Allow HTML formatting in message
• Add admin toggle to enable/disable
• Test message rendering and safety

Labels: enhancement, ui/ux
Priority: Low