feat(05-01): enrich BACKEND_REGISTRY and update all consumers

- Change BACKEND_REGISTRY shape from Record<BackendType, FieldDef[]> to
  Record<BackendType, { displayName, description, fields: FieldDef[] }>
- Add displayName and description to azureblob, s3, s3-compatible entries
- Update schemas/index.ts buildZodSchema to use .fields access
- Update RemoteConfigStep.tsx two .fields accesses (azureblob.find + map)

src/components/wizard/RemoteConfigStep.tsx

@@ -55,7 +55,7 @@ export function RemoteConfigStep() {
           <>
             {/* Account field via FieldRenderer */}
             <FieldRenderer
-              field={BACKEND_REGISTRY.azureblob.find(f => f.key === 'account')!}
+              field={BACKEND_REGISTRY.azureblob.fields.find(f => f.key === 'account')!}
               register={register}
               error={errors.account as FieldError | undefined}
             />
@@ -70,7 +70,7 @@ export function RemoteConfigStep() {
           </>
         ) : (
           /* S3 and S3-compatible: full registry loop — FieldRenderer handles provider hiding */
-          BACKEND_REGISTRY[backendType].map(field => (
+          BACKEND_REGISTRY[backendType].fields.map(field => (
             <FieldRenderer
               key={field.key}
               field={field}