fix(translate): fix batch sizing — use real available input budget, respect job.batch_size, lazy playwright in docker

- _batch_sizer.py: compute per_batch_budget from actual available_input_budget
  (context_window - max_output_tokens) instead of sum of first N rows
- _batch_sizer.py: cap max_rows_hard_cap with job.batch_size (user config)
- _token_budget.py: add available_input_budget and max_output_tokens to return
- preview.py: validate required config fields before preview
- requirements-docker.txt: add playwright pip package (~5 MB)
- backend.entrypoint.sh: lazy playwright install chromium on first start
- build.sh: switch tar to tar.xz (-T0 -9), 5.5x smaller bundles
- README.md: add offline bundle deployment instructions
- playwright.config.js: add testMatch for *.e2e.js files
- frontend: preview tab config validation, i18n keys, translationRun store

backend/requirements-docker.txt

@@ -54,4 +54,7 @@ email-validator
 openai
 tenacity
 Pillow
+# playwright — Python package (~5 MB); Chromium browser (~377 MB)
+# скачивается лениво при первом запуске контейнера через entrypoint
+playwright
 ruff>=0.11.0