Add Cockpit external task instance tab

[hauptmedia]

Summary

• add an external-task API resource and state signal for process instance scoped external tasks
• replace the process instance External Tasks placeholder with a live table and history-mode fallback
• add i18n labels and coverage for the new route behavior

Verification

• node -e "JSON.parse(require('fs').readFileSync('public/locales/en-US/translation.json','utf8')); JSON.parse(require('fs').readFileSync('public/locales/de-DE/translation.json','utf8'));"
• npx eslint src/api/engine_rest.jsx src/api/resources/external_task.js src/api/resources/external_task.test.js src/pages/Processes.jsx src/pages/Processes.test.jsx src/state.js src/state.test.js
• npx vitest run --maxWorkers=4
• npm run build
• browser smoke at http://127.0.0.1:5176/processes/proc:1/instances/inst-9999/external_tasks: reached login boundary with no console errors

[sonarqubecloud]

Quality Gate failed

Failed conditions
9.1% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud