Is it permissible to perform legitimate acts such as remembrance (dhikr), Dhuha prayer, and reading the Qur’anic exegesis during official working hours? Is the one who does so sinful for occupying time that he does not own? Will he be rewarded for it? And what is the ruling if there is no work for the institution?

A government or private employee is a special hire (ajeer khass). Their time during working hours is considered the property of the employer, so it is not permissible to spend it on anything other than the work's interest, if there is work to be done. If there is no work, there is no objection to performing some tasks that do not interfere with the duty, such as Duha prayer and reading. The employer is entitled to the benefit of the special hire throughout the agreed-upon period, with the exception of performing the five obligatory prayers along with their confirmed Sunnah prayers, by custom.