Define draft JSON viewer configuration schema

**Context** We are transitioning ProtVista from hardcoded API logic to a fully configuration-driven architecture. **Task** Draft the first version of the JSON configuration schema that will orchestrate the viewer. This draft needs to explicitly separate the viewer state (Intent) from the data payloads (Representation). **Scope of schema should cover:** * Categories * Tracks (and their order) * Data sources (e.g., URL endpoints, local files, inline data) * Basic rendering options **Notes:** Please keep the "80/20 rule" in mind during drafting. The schema should handle 80% of standard visualization needs declaratively, while ensuring we leave escape hatches (modular APIs) for advanced users to inject custom logic for the remaining 20%.