Let [.] denote the greatest integer function. If the domain of the function \( f(x)=\sin^{-1}\!\left(\frac{x+[x]}{3}\right) \) is \( [\alpha,\beta] \), then \( \alpha^{2}+\beta^{2} \) is equal to:
\( 2 \)
\( 5 \)
\( 10 \)
\( 13 \)