refactor: centralize DIFY settings in config.py and cleanup env files

- Update config.py to read both .env and .env.local (with .env.local priority)
- Move DIFY API settings from hardcoded values to environment configuration
- Remove unused PADDLEOCR_MODEL_DIR setting (models stored in ~/.paddleocr/)
- Remove deprecated argostranslate translation settings
- Add DIFY settings: base_url, api_key, timeout, max_retries, batch limits
- Update dify_client.py to use settings from config.py
- Update translation_service.py to use settings instead of constants
- Fix frontend env files to use correct variable name VITE_API_BASE_URL
- Update setup_dev_env.sh with correct PaddlePaddle version (3.2.0)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/services/translation_service.py

@@ -19,12 +19,11 @@ from app.schemas.translation import (
     TranslationProgress,
     TranslationStatusEnum,
 )
+from app.core.config import settings
 from app.services.dify_client import (
     DifyClient,
     DifyTranslationError,
     get_dify_client,
-    MAX_BATCH_CHARS,
-    MAX_BATCH_ITEMS,
 )
 
 logger = logging.getLogger(__name__)
@@ -205,8 +204,8 @@ class TranslationBatch:
         """Check if item can be added to this batch"""
         item_chars = len(item.content)
         return (
-            len(self.items) < MAX_BATCH_ITEMS and
-            self.total_chars + item_chars <= MAX_BATCH_CHARS
+            len(self.items) < settings.dify_max_batch_items and
+            self.total_chars + item_chars <= settings.dify_max_batch_chars
         )
 
     def add(self, item: TranslatableItem):
@@ -324,7 +323,7 @@ class TranslationService:
 
         logger.info(
             f"Created {len(batches)} batches from {len(items)} items "
-            f"(max {MAX_BATCH_CHARS} chars, max {MAX_BATCH_ITEMS} items per batch)"
+            f"(max {settings.dify_max_batch_chars} chars, max {settings.dify_max_batch_items} items per batch)"
         )
 
         return batches