feat: configurable DTO login/password per KKT device

- Equipment page: add login/password fields in KKT fiscal mode form
- Stored in device config JSON (dto_user, dto_pass)
- Backend passes credentials to agent in test_device command
- Agent uses per-device credentials for all ATOL DTO API calls

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

backend/src/routes/workstations.ts

@@ -300,6 +300,8 @@ const workstationRoutes: FastifyPluginAsync = async (fastify) => {
             com_port: device.com_port,
             fiscal_mode: device.fiscal_mode,
             dto_port: device.dto_port,
+            dto_user: device.config?.dto_user,
+            dto_pass: device.config?.dto_pass,
           },
         }) as { ok: boolean; error?: string; message?: string }
         return result