feat: Hardcode API URL and simplify configuration v1.5.0

Simplification:
- Removed API Endpoint field from settings page
- Users only need to enter API token
- Cleaner, simpler setup process

Hardcoded API URLs:
- Error handler: https://error-vault.com/api/v1/errors
- Backup manager: https://error-vault.com (base URL)
- Health monitor: https://error-vault.com
- Dashboard link: https://error-vault.com/dashboard

Changes:
- Removed api_endpoint from settings sanitization
- Removed api_endpoint input field from admin page
- Hardcoded get_endpoint() in ErrorVault_API
- Hardcoded get_api_base() in EV_Backup_Manager
- Removed api_endpoint checks in health monitor
- Removed api_endpoint check in backup helpers
- Updated dashboard link to use hardcoded URL

Benefits:
- Simpler setup (one field instead of two)
- No risk of incorrect API URL
- Consistent across all installations
- Less configuration errors
- Cleaner admin interface