fix: restore credentials section and add missing User migration

- Use useAuth() in CredentialTable instead of a separate /api/users/ fetch
  that was failing silently and hiding the entire credentials section
- Add migration for autoBackupEnabled/usbBackupEnabled columns on User
  that were in the Prisma schema but missing from the database, causing
  a 500 error on login

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/db/prisma/migrations/20260416000000_add_user_backup_fields/migration.sql

@@ -0,0 +1,3 @@
+-- AlterTable
+ALTER TABLE "User" ADD COLUMN IF NOT EXISTS "autoBackupEnabled" BOOLEAN NOT NULL DEFAULT true;
+ALTER TABLE "User" ADD COLUMN IF NOT EXISTS "usbBackupEnabled" BOOLEAN NOT NULL DEFAULT false;