Company fields that can be left empty now show a "Visible to all" placeholder across several Odoo apps. This helps users understand that records without a specific company are shared across all companies, reducing confusion in multi-company environments.

### Purpose:
Explain to users that when the field is not set, the record is shared across all companies

### After this PR:
Added 'Visible to all' placeholder in company_id fields where,
- its not a required field,
- readonly="False" if have a default value or related/computed field

Task-4159940

The contract signing flow now shows clearer guidance when preparing documents for signature. The update simplifies the screen by replacing an upload button with text and removing an unnecessary warning icon, making the process less distracting for users.

- Updated the warning message.
- Replaced the upload button with a text element.
- Removed the warning icon.

task-4453468

The WhatsApp integration no longer shows a toaster notification when someone leaves a discussion channel. This reduces unnecessary alerts and keeps the user experience cleaner during channel membership changes.

Enterprise counter-part.

Discussed as part of task-4488437

https://github.com/odoo/odoo/pull/195008

This update adjusts tests after required full-time hours became calculated automatically. It helps keep working schedule checks aligned with the updated behavior and reduces false test failures.

- AS `full_time_required_hours` became computed the test should be adabted

Task: 4313444