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).

services/translation_service.py

@@ -1207,28 +1207,55 @@ class TranslationService:
         """
         Translate multiple text strings efficiently using batch processing.
 
+        Deduplicates non-empty texts before sending them to the provider so that
+        repeated strings (common in documents with headers, formulas, tables)
+        are translated only once. This reduces API calls, cost and latency.
+
         Args:
             texts: List of texts to translate
             target_language: Target language code
             source_language: Source language code (default: 'auto')
 
         Returns:
-            List of translated texts
+            List of translated texts in the same order as input
         """
         if not texts:
             return []
 
-        # Use provider's batch method if available
-        if hasattr(self.provider, "translate_batch"):
-            return self.provider.translate_batch(
-                texts, target_language, source_language
-            )
+        # Preserve original positions and filter out empty/whitespace-only texts
+        results: list[str] = [""] * len(texts)
+        unique_texts: list[str] = []
+        text_to_positions: dict[str, list[int]] = {}
 
-        # Fallback to individual translations
-        return [
-            self.translate_text(text, target_language, source_language)
-            for text in texts
-        ]
+        for idx, text in enumerate(texts):
+            if not text or not text.strip():
+                results[idx] = text if text else ""
+                continue
+            if text not in text_to_positions:
+                text_to_positions[text] = []
+                unique_texts.append(text)
+            text_to_positions[text].append(idx)
+
+        if not unique_texts:
+            return results
+
+        # Translate unique texts only
+        if hasattr(self.provider, "translate_batch"):
+            translated_unique = self.provider.translate_batch(
+                unique_texts, target_language, source_language
+            )
+        else:
+            translated_unique = [
+                self.translate_text(text, target_language, source_language)
+                for text in unique_texts
+            ]
+
+        # Map translated texts back to their original positions
+        for original, translated in zip(unique_texts, translated_unique):
+            for idx in text_to_positions[original]:
+                results[idx] = translated if translated else original
+
+        return results
 
 
 # Global translation service instance