[19.0][IMP] contract: label the date fields "Start Date" and "End Date"

[bosd]

Problem

date_start and date_end on contract.recurring.mixin carry no string, so Odoo derives the label from the field name. Every contract, contract line, contract template and contract template line form therefore reads "Date Start" and "Date End".

That is the wrong word order in English, and it is not what Odoo does with the same pair of fields where it defines them itself:

# fleet/models/fleet_vehicle_assignation_log.py
date_start = fields.Date(string="Start Date")
date_end = fields.Date(string="End Date")

# event/models/event_event.py
date_end = fields.Datetime(string='End Date', ...)

Change

Set string="Start Date" / string="End Date" on the mixin, so all four models that inherit it pick the labels up in one place.

Three spots spelled the old wording out by hand and are updated to match:

• the Date End group-by filter on the contract search view
• the Date Start column header in the contract PDF report
• Contract Date Start in the contract reference mail template

Labels only - no behaviour change, no field renames, no view restructuring.

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Second commit: the columns could not be hidden either

Beyond the wording, neither date could be controlled from the line lists:

• date_end carried no optional attribute, so it sat outside Odoo's column selector and was always shown, with no way to switch it off.
• date_start was column_invisible="True", so it was never shown and could not be switched on.

The recurring line list is the exception: date_start is required="1" there and stays outside the selector, since hiding a required column would leave no way to fill it in an editable list.