refactor: update g coupling to use capacitance matrix formula

[shanto268]

Summary

• Updated qubit-resonator coupling strength (g) calculation to use the capacitance matrix formalism
• Formula: g = (C_g / sqrt(C_Q + C_g)) * sqrt(hbar * omega_r * e^2 / det(C)) * (E_J / (8 * E_C,Q))^(1/4)
• Updated g_from_cap_matrix(), g_from_cap_matrix_numba(), calculate_target_quantities(), and find_g_a_fq()

[ykw1793]

Ec is computed using Ec_from_Cs function which uses the approximated expression for Ec. The unapproximated expression is

Ec = e^2 / (2 * (det_C / (C_r + C_g)))