Correct linking of FFTW3 lib

[mmejiam-eafit]

Implement these changes:

# ── System dependencies ──────────────────────────────────────────────────────
find_package(PkgConfig REQUIRED)
pkg_check_modules(FFTW3 REQUIRED IMPORTED_TARGET fftw3)

And also these changes:

# ── Domain library (pure physics, no rendering) ────────────────────────────
add_library(quantum_domain STATIC
    src/domain/wavefunctions/WaveFunction1D.cpp
    src/domain/wavefunctions/WaveFunction2D.cpp
    src/domain/wavefunctions/WaveFunction3D.cpp
    src/domain/wavefunctions/SpinorWaveFunction1D.cpp
    src/domain/potentials/Potential.cpp
    src/domain/potentials/BarrierPotential.cpp
    src/domain/potentials/HarmonicPotential.cpp
    src/domain/potentials/DoubleSlitPotential.cpp
    src/domain/particles/Particle.cpp
    src/domain/particles/Photon.cpp
    src/domain/interactions/CoulombInteraction.cpp
    src/domain/interactions/ScatteringEngine.cpp
)
target_include_directories(quantum_domain PUBLIC src)
target_link_libraries(quantum_domain PUBLIC glm::glm PkgConfig::FFTW3)
target_link_libraries(quantum_domain PRIVATE project_warnings)