Komentaryo sa Genesis 49:24: Rashi, Chizkuni, Ramban & Rashbam

ותשב באיתן קשתו means his power (i.e. his rule as viceroy) was strongly (באיתן) established.

ותשב באיתן קשתו, “Yet his bow stayed in its original position;” Yaakov illustrates how Joseph restrained himself not taking revenge on his brothers when he had the opportunity to do so.

BY THE HANDS OF THE MIGHTY ONE OF JACOB. This befell him from the Holy One, blessed be He, Who is the Mighty One of Jacob. And from there [from his position as viceroy], he became worthy to be the feeder of the foundation stone of Israel, that is, the source of the tribes of Israel. The word “even” (stone) has the same meaning here as the word “even” in the expression, the chief stone,²²⁹Zechariah 4:7. [where it means high position]. This is the language of Rashi. “From the G-d of thy father did this befall you, and He will help you. [And with the Almighty, i.e.,] and your heart was with the Holy One, blessed be He, when you did not hearken to the words of your mistress, and He will bless you.” This is also the language of Rashi.
Rabbi Abraham ibn Ezra said that the letter mem in the expression, mei’e-il avicha (‘from’ the G-d of thy father), is also to apply to the expression, ve’eith Sha-dai, [as if it read, umei’eith Sha-dai (and ‘from’ the Almighty)], and He will bless thee.…

קשתו means his power.