fix: prevent custom registry org input from deselecting

The ImageRegistryConfig component's key included registry.organization,
which caused React to remount the component on every keystroke when
typing a custom organization name. This destroyed the OrganizationSelector's
internal customMode state, causing the input to deselect after each character.

Using a stable index-based key preserves component identity during edits.

Closes #1094

frontend/src/components/resources/build/config.tsx

@@ -266,12 +266,7 @@ export const BuildConfig = ({
 
             {image_registries?.map((registry, index) => (
               <ImageRegistryConfig
-                key={
-                  (registry.domain ?? "") +
-                  (registry.organization ?? "") +
-                  (registry.account ?? "") +
-                  index
-                }
+                key={index}
                 registry={registry}
                 imageName={imageName}
                 setRegistry={(registry) =>