Refactor processing.ts to remove magic strings for modifier names

[andeplane]

Refactor src/store/processing.ts to remove magic strings for modifier names (e.g., "Particles", "Bonds", "Colors").

Suggestions:

• Define a constant array RENDERING_MODIFIER_NAMES
• Or add a property to the Modifier class to identify its category (e.g., category: 'rendering').

See PR #210 for context.