View validation has been streamlined so Odoo can check screens and customizations more efficiently. This should reduce delays during updates or configuration changes while keeping existing Gantt, Grid, and Studio behavior aligned with the core platform changes.

See https://github.com/odoo/odoo/pull/75796

The planning Gantt view now makes employee availability easier to understand. When an employee is not expected to work during a period, the progress bar is replaced with a warning icon and explanatory message, reducing confusion for schedulers.

This PR improves planning's gantt view's progress bar:

- Prevents the `lightened` effect introduced in PR #18045 on .o_gantt_row_title
- When the employee isn't expected to work, replace the progress bar by an 
  exclamation triangle icon and tell the user the employee isn't expected to 
  work during this period


task-2638342