test(core): replace image-cache module mocks with globalThis.fetch spies

Switch `mock.module("../src/image-cache", ...)` stubs in thumbhash, person-detail, colors, and credits tests to `spyOn(globalThis, "fetch")` instead. This avoids the cross-file persistence problem with `mock.module` and tests the real code paths more faithfully.

Update `image-cache.ts` to call `globalThis.fetch` so the spy intercepts it, add `mock.restore()` to the global `afterEach` in `preload.ts`, and set `IMAGE_CACHE_ENABLED=false` in affected tests to avoid disk writes during test runs.

packages/core/src/image-cache.ts

@@ -63,7 +63,7 @@ async function fetchRemoteImage(
   const url =
     tmdbCdnImageUrl(tmdbPath, category) ?? `${TMDB_IMAGE_BASE_URL}${tmdbPath}`;
 
-  const res = await fetch(url);
+  const res = await globalThis.fetch(url);
   if (!res.ok) {
     log.warn(`Fetch failed: ${url} -> ${res.status}`);
     return null;