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).

frontend/src/lib/i18n/messages/en/admin.json

@@ -0,0 +1,48 @@
+{
+  "admin.login.title": "Administration",
+  "admin.login.required": "Login required",
+  "admin.login.password": "Admin password",
+  "admin.login.connecting": "Connecting...",
+  "admin.login.access": "Access admin panel",
+  "admin.login.restricted": "Restricted to administrators",
+  "admin.layout.checking": "Verifying authentication...",
+  "admin.dashboard.title": "Admin Dashboard",
+  "admin.dashboard.subtitle": "Administrator control panel",
+  "admin.dashboard.refresh": "Refresh",
+  "admin.dashboard.refreshTooltip": "Refresh dashboard data",
+  "admin.dashboard.config": "System Configuration",
+  "admin.dashboard.maxFileSize": "Max file size:",
+  "admin.dashboard.translationService": "Translation service:",
+  "admin.dashboard.formats": "Formats:",
+  "admin.nav.dashboard": "Dashboard",
+  "admin.nav.users": "Users",
+  "admin.nav.pricing": "Pricing & Stripe",
+  "admin.nav.providers": "Providers",
+  "admin.nav.system": "System",
+  "admin.nav.logs": "Logs",
+  "admin.users.title": "User Management",
+  "admin.users.subtitle": "View and manage user accounts",
+  "admin.users.planUpdated": "Plan updated",
+  "admin.users.planChanged": "The plan has been changed to \\\"{plan}\\\" successfully.",
+  "admin.users.unknownError": "Unknown error",
+  "admin.users.error": "Error",
+  "admin.users.planUpdateError": "Unable to update plan: {message}",
+  "admin.users.noKeys": "No keys",
+  "admin.users.noKeysDesc": "This user has no active API keys.",
+  "admin.users.keysRevoked": "Keys revoked",
+  "admin.users.keysRevokedDesc": "{count} API key(s) revoked successfully.",
+  "admin.users.revokeError": "Unable to revoke keys: {message}",
+  "admin.users.retry": "Retry",
+  "admin.system.title": "System",
+  "admin.system.subtitle": "Monitor system status and manage resources",
+  "admin.system.quotas": "Translation quotas",
+  "admin.system.resetQuotas": "Reset monthly quotas",
+  "admin.system.resetting": "Resetting...",
+  "admin.system.reset": "Reset",
+  "admin.system.allOperational": "All Systems Operational",
+  "admin.system.issuesDetected": "System Issues Detected",
+  "admin.system.waitingData": "Waiting for data...",
+  "admin.system.purging": "Purging...",
+  "admin.system.clean": "Clean",
+  "admin.system.purge": "Purge"
+}