feat(web): tint objects by their ColorDiffuse

Model ColorDiffuse in shared types and multiply each viewer's material color by the per-object tint, including textured faces and loaded custom models.

packages/shared/src/types.ts

@@ -30,6 +30,8 @@ export interface TTSObject {
   Parent?: TTSObject;
   /** Placement in the TTS world (position, rotation, scale). */
   Transform?: TTSObjectTransform;
+  /** Tint color (0–1 per channel) applied to the object's base color. */
+  ColorDiffuse?: { r: number; g: number; b: number };
 
   CustomPDF?: {
     PDFUrl: string;