The appointment booking calendar now guides visitors to available time slots more clearly, including helpful messages when no slots are available and links to the next action. Logged-in users creating appointment types also get better setup support, such as default employee selection and clearer explanations when availability is missing.

In master, the calendar UI to register for an appointment is unclear.

In this commit, we improve the design for visitors in a few ways:

 - When landing on the calendar page of an appointment type, the first
available slot is selected for the visitor, instead of potentially
leaving them on an empty month.
When switching months, the selected slot (if any) is cleared to avoid
confusion.
 - When a month has no available slot, a descriptive message is shown
on the right side, with a relevant icon, and a link to the first
available slot.
 - When an appointment type has no available slot altogether, a message
is also displayed on the right side, with a relevant icon, and a link
to the 'contact us' page.

We also improve the experience of logged-in users:

 - When creating an appointment type from the front end, the employees
can be picked directly, and the current user is added to the list by
default (but can be removed if needed, of course).
 - When an appointment type has no available slot, a relevant message
is shown on top of the calendar, specifying why there aren't any 
(no employee, no availabilities configured,...). A link to the back
end is also added.

task-2501575

Employees can now cancel past time off when payroll is installed, provided no validated payslip covers that period and they had an active contract during the leave. This gives HR and employees more flexibility to correct past absences while preserving payroll validation controls.

Prior to this commit there was no way for the employee to cancel a leave
in the past.
However when payroll is installed, it should be possible as long as no
payslip has been validated for that period.
It will now be possible to do that as long as the employee has a
contract active for the time off's period.
Otherwise the old rules still apply.

TaskId-2792249

The spreadsheet component used in Documents has been updated to a newer version. This keeps the embedded spreadsheet experience aligned with the latest improvements and helps maintain compatibility with newer underlying interface technology.

### Contains the following commits:

https://github.com/odoo/o-spreadsheet/commit/8627134e [IMP] package: update owl to 2.0-beta.3

This fix ensures currency data is only loaded once when opening or using bundled spreadsheets. It avoids unnecessary repeated processing, helping spreadsheet features behave more consistently and efficiently.

Before this commit, the "load_currency" could be triggered more than once
as we returned an empty answer.

Task-id 2796425

This update reorganizes part of the enterprise messaging interface code without changing visible features. It helps keep the underlying component structure cleaner and easier to maintain, reducing future development risk.

Task-2759571

Steps : Install Rental and Inventory. Inventory > Settings > Traceability > Enable Lots & S/N. Rental > Create a Product: > Invetory > Tracking : By Unique S/N Create a Rental Order and add this Product to the Order Lines. In the wizard that pops up, set the same dates for "from" and "to" dates and click on Apply. Issue : TypeError: can only concatenate list (not "sale.order.line") to list. Cause : Clicking on Apply calls _get_unavailable_qty_and_lots(), which calls _get_active_

Steps :
Install Rental and Inventory.
Inventory > Settings > Traceability > Enable Lots & S/N.
Rental > Create a Product:
	> Invetory > Tracking : By Unique S/N
Create a Rental Order and add this Product to the Order Lines.
In the wizard that pops up, set the same dates for "from" and "to"
dates and click on Apply.

Issue :
TypeError: can only concatenate list (not "sale.order.line") to list.

Cause :
Clicking on Apply calls _get_unavailable_qty_and_lots(),
which calls _get_active_rental_lines().
When "from" and "to" are equal, _get_active_rental_lines()
returns 2 empty lists and some SOLs.
Later, we try to concatenate those results together.

Fix :
Instead of returning empty lists, return empty records.

opw-2793666

Forward-Port-Of: odoo/enterprise#25442

Forward-Port-Of: odoo/enterprise#25422

Forward-Port-Of: odoo/enterprise#25422