fix: minor fixes and template updates

- seed_demo.py: Change company owner role from "user" to "vendor"
- header.html: Update header partial styling/content
- marketplace.js: Minor JS updates
- marketplace.html: Template updates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

scripts/seed_demo.py

@@ -315,7 +315,7 @@ def create_demo_companies(db: Session, auth_manager: AuthManager) -> list[Compan
                 hashed_password=auth_manager.hash_password(
                     company_data["owner_password"]
                 ),
-                role="user",
+                role="vendor",
                 first_name=company_data["owner_first_name"],
                 last_name=company_data["owner_last_name"],
                 is_active=True,