The web interface color palette has been refreshed to use the calendar view colors as the default across the Odoo UI. This helps make the visual experience more consistent for users while keeping the change focused on appearance rather than functionality.

Use calendar_view colours as default odoo ui colours.

part of: https://github.com/odoo/enterprise/pull/5335
task: [1966473](https://www.odoo.com/web#id=1966473&model=project.task) 




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This update restructures the project timesheet and employee leave integration to make it easier to maintain and extend. It addresses existing maintenance needs without introducing notable changes for day-to-day users.

Description of the issue/feature this PR addresses:
Addresses #30854 and regular code maintenance.

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This update streamlines how project timesheets linked to time off handle date and time values. It removes an unnecessary conversion step, which helps keep the code cleaner and reduces the chance of date handling inconsistencies without changing user-facing behavior.

Description of the issue/feature this PR addresses:
No need to convert `datetime` to string

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The Gantt planning views have been visually refreshed with cleaner styling, updated colors, and improved display of time-off information such as totals and user thumbnails. This makes scheduling screens easier to read and more consistent with the broader Odoo interface.

needs:  https://github.com/odoo/odoo/pull/36218
task: [1966473](https://www.odoo.com/web#id=1966473&model=project.task)