Associahedra for finite type cluster algebras and minimal relations between $\mathbf{g}$-vectors

We show that the mesh mutations are the minimal relations among the $\boldsymbol{g}$-vectors with respect to any initial seed in any finite type cluster algebra. We then use this algebraic result to derive geometric properties of the $\boldsymbol{g}$-vector fan: we show that the space of all its polytopal realizations is a simplicial cone, and we then observe that this property implies that all its realizations can be described as the intersection of a high dimensional positive orthant with well-chosen affine spaces. This sheds a new light on and extends earlier results of N. Arkani-Hamed, Y. Bai, S. He, and G. Yan in type $A$ and of V. Bazier-Matte, G. Douville, K. Mousavand, H. Thomas and E. Yildirim for acyclic initial seeds. Moreover, we use a similar approach to study the space of polytopal realizations of the $\boldsymbol{g}$-vector fans of another generalization of the associahedron: non-kissing complexes (a.k.a. support $\tau$-tilting complexes) of gentle algebras. We show that the space of realizations of the non-kissing fan is simplicial when the gentle bound quiver is brick and $2$-acyclic, and we describe in this case its facet-defining inequalities in terms of mesh mutations. Along the way, we prove algebraic results on $2$-Calabi-Yau triangulated categories, and on extriangulated categories that are of independent interest. In particular, we prove, in those two setups, an analogue of a result of M. Auslander on minimal relations for Grothendieck groups of module categories.