fix: correct Fourier transform sign in k→R density matrix transform

[dyzheng]

The inverse Fourier transform from k-space to real-space density matrix used e^{+ikR} instead of the correct e^{-ikR} as specified in nao_lcao_force_stress_derivation.md Section 3.

This caused the stored DMR to be D_{μν}(-R) instead of the required trace-DM D̄_{μν}(R) = D_{νμ}(-R), leading to incorrect nonlocal pseudopotential forces for non-collinear (nspin=4) calculations with SOC.

Changes:

• density_matrix.cpp: Fix sign in cal_DMR(), cal_DMR_td(), cal_DMR_full() from TK(cosp, sinp) to TK(cosp, -sinp)
• dmr_complex.cpp: Fix sign in linear-response cal_DMR() specialization

Verified by Hermiticity check D(R) = D†(-R) at machine precision.

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