fix: load product images through the API instead of the storefront origin

[PhilippTheServer]

Closes #8

ApiService.items() and item() now resolve API-relative media paths (main_image, gallery) against apiUrl: /v1/items/{uuid}/image → /api/v1/items/{uuid}/image. Absolute URLs and missing images are untouched. The cart stores the items these calls return, so the shop grid, product page and cart are all fixed. README notes it.

Verification

npx ng test --watch=false (new src/app/core/api.service.spec.ts)

before the fix:
 × resolves API-relative images of a single item against the API
 × resolves images in a catalogue page and leaves absolute or missing ones alone
 Tests  2 failed | 23 passed (25)

after:
 Test Files  3 passed (3)
      Tests  25 passed (25)

Also checked in the browser against the local dev stack (storefront via docker compose, 40 items with uploaded images): images now render on /shop, /shop/:id and /cart; before, every one was broken.

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