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

frontend/playwright.config.js

@@ -13,6 +13,7 @@ const AUTH_PASSWORD = process.env.E2E_PASSWORD || 'admin123';
 
 export default defineConfig({
 	testDir: './e2e/tests',
+	testMatch: '**/*.e2e.js',
 	fullyParallel: false,
 	retries: process.env.CI ? 1 : 0,
 	workers: 1, // sequential tests — settings mutate global state