Add XYZ file support

[maxscheurer]

This PR adds support for loading .xyz molecular geometry files (atom count + comment + element/coordinates), commonly used in computational chemistry.

Other changes:

• XYZ files default to Wireframe mode with Element coloring (since they lack chain/residue/secondary structure info); CLI flags override as usual
• Replace hardcoded 1.9 Å bond distance cutoff with covalent-radii-based detection using 1.3 × (r₁ + r₂) (Cordero et al. radii — fixes missing bonds for metal-ligand complexes and spurious bonds between non-bonded hydrogens, benefiting both XYZ and PDB/mmCIF rendering
• Fix initial camera zoom for XYZ files — bounding_radius() previously only considered backbone atoms, producing an over-zoomed view for files with no backbone