fix: ensure domain and range classes are always visible (fixes #172)

[mithunbabbira]

Problem

WebVOWL treats domain classes differently than range classes. For an ontology like:

prefix : <https://example.com/>

:simple
  rdfs:domain :From;
  rdfs:range :To.

WebVOWL displays the :To (range) class but not the :From (domain) class — unless :From is explicitly declared as a class with :From a owl:Class.

Root Cause

The parser builds a class map from declared classes in the ontology data. When a property references a class ID that's not in the declared classes list, the classMap[] lookup returns undefined, and the property gets silently dropped rather than having its domain node rendered.

Fix

After parsing properties, collect all domain/range class IDs referenced by properties. For any class ID not already in the ontology's class list, create a placeholder OwlClass node and add it to the combined classes array before createNodeStructure() is called.

This ensures both domain and range classes are treated identically — both will always be visible when referenced by a property, regardless of whether they're explicitly declared in the ontology data.

Fixes #172