Add 3D object viewers with r3f stack

Add per-class 3D viewers for tiles, tokens, cards, and custom models
using React Three Fiber, drei, and postprocessing. Viewers are
lazy-loaded and registered through the existing viewer registry, with a
shared scene wrapper for lighting, orbit controls, and subtle effects.

Add a CORS-safe /asset proxy route so three.js loaders can fetch
Workshop-hosted textures and models, and extend TTSObject with the
CustomMesh and CustomTile/CustomToken fields the viewers read.

apps/web/src/components/viewers/assetUrl.ts

@@ -0,0 +1,8 @@
+/**
+ * Route an external asset URL through the proxy so it can be loaded by
+ * three.js loaders (TextureLoader, GLTFLoader, etc.) despite the upstream host
+ * omitting CORS headers.
+ */
+export function assetUrl(url: string): string {
+  return `/asset?url=${encodeURIComponent(url)}`;
+}