[FIX] web: CalendarView: event edition in month mode

Fixes the way record end hours are computed in calendar views.

Before this commit:  objects `record start` and `record end` properties being copied by reference
and being incorrectly rewritten by other functions.
This caused incorrect end hour computations (and a SQL crash in the Planning app).

Now, objects are copied by value and don't interfere with each other.

Task 2072961

closes odoo/odoo#37690

Signed-off-by: Aaron Bohy (aab) <aab@odoo.com>