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/lib/api/client.ts

@@ -118,3 +118,17 @@ export async function upload<T>(
     body: form,
   })
 }
+
+export async function uploadWithFields<T>(
+  path: string,
+  file: File,
+  fields: Record<string, string>,
+): Promise<T> {
+  const form = new FormData()
+  form.append("file", file)
+  for (const [k, v] of Object.entries(fields)) form.append(k, v)
+  return request<T>(buildUrl(path), {
+    method: "POST",
+    body: form,
+  })
+}