Add comprehensive demo data loading logic and .env.demo configuration

- Implemented `load_demo_data` to populate organizations, users, and relationships from `demo_data.json`.
- Refactored database initialization to handle demo-specific passwords and multi-entity creation in demo mode.
- Added `demo_data.json` with sample organizations and users for better demo showcase.
- Introduced `.env.demo` to simplify environment setup for demo scenarios.
- Updated `.gitignore` to include `.env.demo` while keeping other `.env` files excluded.

frontend/.gitignore

@@ -33,6 +33,7 @@ yarn-error.log*
 
 # env files (can opt-in for committing if needed)
 .env*
+.
 
 # vercel
 .vercel