feat: JSON import + mapping presets in SPA

- Accept .json files in import upload, send format to backend
- Backend endpoint PUT /import/sessions/{id}/profile/{profile_id}
- Load saved presets on mapping step, auto-apply and skip to preview
- Save current mapping as preset on confirm step
- Delete presets from mapping step

spa/src/hooks/use-imports.ts

@@ -1,5 +1,6 @@
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query"
 import {
+  applyImportProfile,
   applyMapping,
   confirmImport,
   createImportSession,
@@ -93,3 +94,10 @@ export function useDeleteImportProfile() {
     },
   })
 }
+
+export function useApplyImportProfile() {
+  return useMutation({
+    mutationFn: ({ sessionId, profileId }: { sessionId: string; profileId: string }) =>
+      applyImportProfile(sessionId, profileId),
+  })
+}