Is dissatisfaction with one's current job and seeking another considered a lack of contentment with divine decree and destiny, or a violation of Islamic law?

What the questioner mentioned is not an objection to Allah’s decree. For a person to search for suitable work that they can master and through which they can benefit Muslims is something مطلوب (desirable) by religious law and by reason, and it is part of Allah’s decree. The intelligent believer flees from a decree that is unsuitable for him to a decree that is suitable for him, just as Umar ibn al-Khattab (may Allah be pleased with him) did when he fled from the plague in ash-Sham. The believer is keen on what benefits him and knows that he will not go outside of Allah’s decree, seeking help from Him, the Exalted, as the Prophet (peace be upon him) said: "Be keen on what benefits you, and seek help from Allah and do not be helpless." Thus, the believer combines being keen on what benefits him and seeking help from Allah, with his knowledge that whatever will be is by Allah’s decree. As for the questioner’s current work, he must fear Allah in it and master it and fulfill the terms of his contract, and there is no harm upon him in searching for another job.