Am I entitled to demand the return of the mobile phone that was stolen from me after I gave it to someone who asked me for help in obtaining qat, and is this considered theft?

First, chewing qat is forbidden (haram).

Regarding the person who took your mobile phone and denied it, you have the right to file a complaint against him with the police.

Scholars have differed on the ruling concerning a person who denies a borrowed item: should his hand be cut off like a thief, or should he be subjected to discretionary punishment (ta'zir) like a betrayer?

The majority of scholars—including the three Imams Abu Hanifa, Malik, and Al-Shafi'i—are of the opinion that his hand should not be cut off, citing the hadith: "There is no cutting off for a betrayer, nor for a plunderer, nor for a swindler."

While others, such as the Hanbalis, Ishaq, and the Zahiris, hold that his hand should be cut off, citing the hadith of Aisha (may Allah be pleased with her) that a woman used to borrow items and deny them, so the Prophet (peace be upon him) commanded that her hand be cut off.

This latter opinion is the more preponderant (rajih), because the harm caused by a person who denies a borrowed item can be greater than that of a thief, and to protect people's properties and to encourage acts of kindness between the lender and the borrower.