**Description of the issue/feature this PR addresses:**

When adding a custom field (e.g., via Studio) to the Documents list view, clicking the cell directly does not trigger inline edit mode. The user has to first click a standard editable field (like "Owner") to put the row into edit mode before they can modify the custom field.

This occurs because we use a hardcoded whitelist (`editableColumns`) of standard fields allowed to trigger edit mode. Custom fields (`x_`) are missing from this static list.

This commit resolves the issue by dynamically injecting visible, non-readonly custom fields into the `editableColumns` whitelist. This allows user-created fields to be edited inline as expected.

**Steps to reproduce:**

- Documents > Studio > List view > Add any field that accepts user input (e.g. Text/char) > save/exit
- In the same Documents list view > select a row > click the cell belonging to the newly created field > observe that the row does not enter edit mode
- In the same Documents list view > select a row > click a standard editable cell, then click the cell belonging to our newly created field > observe that this then allows us to edit our field

**Current behavior before PR:**

- Custom fields do not trigger inline edit mode

**Desired behavior after PR is merged:**

- Custom fields trigger inline edit mode

opw-6378102

Forward-Port-Of: odoo/enterprise#125239

purpose:
1- We should have a consistent way to compute `_gantt_unavailability` of employees in time off and attendance. Currently, some cases have inconsistent behavior such as out of contract days, flexible and fully flexibe employees.
2- In time off calendar view, if the employee does not have a contract at all, the current working schedule will appear in the calendar and it will not be greyed out. This is inconsistent with the behavior of the attendance application.

Fix:
1. 
- implemented `_get_employee_unavailable_intervals` in employee model to be used in both time off and attendance.
    - more optimized than the old implementation in time off as it calls `_work_intervals_batch` once per calendar instead of calling it for each contract in `_unavailable_intervals_batch`
    - greys out "out of contract" periods
    - for flexible and fully flexible employees, the whole period is considered available except leave periods
- made `_get_calendar_periods` use version date start instead of contract date start and corrected a bug in tz conversion
2. 
- made `_get_unusual_days` return True for all the days outside of contracts for the employee instead of not returning anything for them or getting values from the working schedule of the employee (means that they will be greyed out in the callendar view) and added a test for it

task-id: 5473055

Steps to reproduce:
- set the company currency to XCG
- set the PoS sales journal currency and the PoS pricelist currency to USD
- configure a XCG <-> USD rate
- create a customer without any outstanding balance
- open the PoS, create an order of USD 100 and validate it with the Customer Account (Pay Later) payment method
- open the Customers screen and look at the Total Due of that customer

Issue:
The Total Due shows about USD 55.56, i.e. the amount converted once too many, instead of the expected USD 100.

Cause:
get_total_due() sums two amounts that are not expressed in the same currency before converting them. partner.total_due comes from the accounting entries, it is the sum of account.move.line.amount_residual and is therefore in company currency, while total_settled is the sum of pos.payment.amount of the still open sessions, which is in the currency of the order, so the PoS one. The addition is done first and the result is then converted from the company currency to the PoS one, so the pay later payments end up converted a second time.

opw-6403320

Forward-Port-Of: odoo/enterprise#126111
Forward-Port-Of: odoo/enterprise#125798

Steps to reproduce:
- Configure a preparation display on a POS config with a product category
- Place an order and leave it in a non-final stage
- Keep the session open past midnight

Issue:
The kanban order-count badge drops the order once its create_date falls behind "today", while the preparation screen still lists it. The same divergence makes the badge keep counting an order that a "Reset" already removed from the screen.

_compute_order_count() scoped its search on pos_config_id and create_date >= today, whereas the screen is built by get_preparation_display_order() from _get_open_orders_in_display() and _get_stageless_orders_in_display(), which have no date filter and instead bound the set by the order stage `done` flag and the session state. An order open across midnight is therefore in the screen set but not in the badge set. Conversely reset() marks the current stage done, which drops the order from the screen set, but the badge only skipped orders whose latest stage is the final stage, so an order reset while still in the first stage stayed counted.

opw-6414302

Forward-Port-Of: odoo/enterprise#126139
Forward-Port-Of: odoo/enterprise#125984

**Problem:**
When an appointment is cancelled, the cancellation email is always sent in the organizer's (staff member's) language, ignoring the booking customer's language. The booking confirmation, by contrast, is correctly localized.

**Steps to reproduce:**
1. Set a contact's language to a non-default one (e.g. Romanian).
2. Book an appointment for that contact (they are the booker/attendee).
3. Cancel the appointment.
4. The customer received the confirmation in Romanian but the cancellation email arrives in English.

**Current behavior:**
The cancellation email is rendered in the organizer's language.

**Expected behavior:**
The cancellation email is rendered in the booking customer's language, like the confirmation/invitation email.

**Cause of the issue:**
The cancellation uses `appointment_canceled_mail_template`, whose `lang` is `{{ object.partner_id.lang }}`. On `calendar.event`, `partner_id` is `related='user_id.partner_id'`, i.e. the organizer, not the customer. The template is posted once per event (via `_track_template`), so its single rendering language applies to every recipient, including attendees whose own language differs. The confirmation email is unaffected because it is the per-attendee `attendee_invitation_mail_template` (model `calendar.attendee`), rendered once per attendee in that attendee's language.

**Fix:**
Deriving the language from `appointment_booker_id` makes the cancellation consistent with the other appointment mails, which are meant for the person who booked the meeting. It falls back to `partner_id` when there is no booker (e.g. an event not created through the appointment flow), preserving the previous behavior in that case.

opw-6323179

Forward-Port-Of: odoo/enterprise#124116