Request Immich preview-size images instead of tiny default thumbnails

The default /thumbnail endpoint serves a 250x374 WebP, which is
visibly blurry on portfolio cards meant to show off real work.
size=preview returns a ~1440px-wide JPEG at a reasonable file size,
confirmed directly against the live Immich instance.

Fixes #25.

src/index.html

@@ -1723,9 +1723,10 @@
           return [];
         }
 
-        // Build thumbnail URLs for all assets
+        // Immich's default thumbnail size is a 250px-wide WebP — too small for
+        // full-bleed portfolio cards. size=preview serves a ~1440px JPEG instead.
         const thumbnailUrls = assets.map(asset =>
-          `${baseUrl}/api/assets/${asset.id}/thumbnail?key=${shareKey}`
+          `${baseUrl}/api/assets/${asset.id}/thumbnail?key=${shareKey}&size=preview`
         );
 
         return thumbnailUrls;