perf+security: fix build, secure downloads, dedupe translations, refactor i18n

Frontend:
- Fix Framer Motion / motion-dom build error by pinning framer-motion to
  11.18.2 (compatible with React 19 and Next.js 16).
- Add cross-env and build:local script to bypass standalone symlink errors
  on Windows without Developer Mode.
- Allow NEXT_OUTPUT=default to disable standalone output for local builds.
- Refactor i18n: split 14,177-line src/lib/i18n.tsx into per-locale,
  per-namespace JSON files under src/lib/i18n/messages/.
- Load English synchronously; other locales loaded on demand via dynamic
  imports (reduces initial bundle, improves maintainability).
- Remove unused next-intl message files src/messages/en.json and fr.json.

Backend:
- Remove insecure legacy /api/v1/download/{filename} and /api/v1/cleanup/{filename}
  endpoints. The job-based /api/v1/download/{job_id} already enforces ownership.
- Deduplicate texts in TranslationService.translate_batch before sending them
  to the provider, reducing API calls for repeated strings.
- Pin httpx to <0.28 to fix TestClient incompatibility with starlette 0.35.1.
- Add pytest-cov and ruff dev dependencies/config.

DevOps:
- Remove hardcoded Grafana password from docker-compose.yml and
  docker-compose.monitoring.yml; use GRAFANA_PASSWORD env var.
- Change default TRANSLATION_SERVICE from ollama to google in
  docker-compose.yml (Ollama is an optional profile).
- Add GRAFANA_PASSWORD to .env.example.
- Add .coverage and frontend/pnpm-workspace.yaml to .gitignore.

Tests:
- Update API versioning tests for removed legacy endpoints.
- Add tests/test_translation_service.py for deduplication behavior.

Verified:
- pnpm run build:local passes.
- uv run pytest tests/test_providers/* tests/test_translation_service.py
  tests/test_story_3_5_api_versioning.py tests/test_download_endpoint.py
  tests/test_translators/test_excel_translator.py: provider/translator tests
  pass; one pre-existing French error-message test still fails (message is
  returned in English, unrelated to this change).

docker-compose.monitoring.yml

@@ -8,7 +8,7 @@
 #   docker compose -f docker-compose.yml -f docker-compose.monitoring.yml up -d
 #
 # Access:
-#   Grafana:  http://<IP_SERVEUR>:3001  (admin / WordlyGrafana2026!)
+#   Grafana:  http://<IP_SERVEUR>:3001  (admin / $GRAFANA_PASSWORD)
 #   Ou via NPM: monitoring.wordly.art -> wordly-grafana:3000
 # ============================================
 
@@ -54,8 +54,8 @@ services:
     container_name: wordly-grafana
     restart: unless-stopped
     environment:
-      - GF_SECURITY_ADMIN_USER=admin
-      - GF_SECURITY_ADMIN_PASSWORD=WordlyGrafana2026!
+      - GF_SECURITY_ADMIN_USER=${GRAFANA_USER:-admin}
+      - GF_SECURITY_ADMIN_PASSWORD=${GRAFANA_PASSWORD}
       - GF_USERS_ALLOW_SIGN_UP=false
       - GF_SERVER_ROOT_URL=https://monitoring.wordly.art
       - GF_INSTALL_PLUGINS=grafana-clock-panel