refactor: share trace, error boundary, and proxy http helpers

Lift the trace-to-shape geometry into @tts/mesh (traceToShape/
traceToUvBounds), parameterized by scale so the web token viewer and
@tts/tabletop share one implementation. Move the CORS proxy HTTP helpers
(assetUrl, resolveAssetUrl, traceImage) into a new @tts/http package, and
make @tts/tabletop's ErrorBoundary the single source used by the web app.

This removes the duplicated ErrorBoundary, assetUrl, tabletopHttp, and
trace-to-shape code from apps/web and packages/tabletop.

packages/http/src/asset.ts

@@ -0,0 +1,26 @@
+/**
+ * Asset URL helpers for the host app's CORS-safe asset proxy (`/asset`).
+ * Shared by the web viewers and `@tts/tabletop`, which both route external
+ * assets (textures, models) through the proxy so three.js can load them.
+ */
+
+/**
+ * Resolve a part's asset URL. Absolute URLs (http/https, data:, blob:) pass
+ * through unchanged; relative paths are resolved against `baseUrl` — the
+ * directory of the part's source file (the json/yaml/markdown that defined
+ * it). Without a `baseUrl`, a relative path is returned as-is.
+ */
+export function resolveAssetUrl(url: string, baseUrl?: string): string {
+  if (/^(https?:|data:|blob:)/i.test(url)) return url;
+  if (!baseUrl) return url;
+  // `baseUrl` is a path relative to the games root (no scheme), so join rather
+  // than resolve as a URL. A fake origin lets `new URL` normalize `./` and
+  // `../`; the origin is stripped from the result.
+  const resolved = new URL(url, `http://localhost/${baseUrl}`).pathname;
+  return resolved.replace(/^\/+/, '');
+}
+
+/** Route an external asset URL through the proxy so three.js can load it. */
+export function assetUrl(url: string): string {
+  return `/asset?url=${encodeURIComponent(url)}`;
+}