feat: add multilingual support to vendor onboarding workflow

- Add language selector with English, French, and German translations
- Fix API key help text to reference Letzshop Support team
- Update shop slug input with URL prefix and clearer example
- Fix step validation bug by adding db.commit() to all POST endpoints
- Translate all form labels, buttons, and messages

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/vendor/onboarding.py

@@ -92,7 +92,7 @@ def save_company_profile(
     Updates vendor and company records with provided data.
     """
     service = OnboardingService(db)
-    return service.complete_company_profile(
+    result = service.complete_company_profile(
         vendor_id=current_user.token_vendor_id,
         company_name=request.company_name,
         brand_name=request.brand_name,
@@ -105,6 +105,8 @@ def save_company_profile(
         default_language=request.default_language,
         dashboard_language=request.dashboard_language,
     )
+    db.commit()  # Commit at API level for transaction control
+    return result
 
 
 # =============================================================================
@@ -142,12 +144,14 @@ def save_letzshop_api(
     Tests connection first, only saves if successful.
     """
     service = OnboardingService(db)
-    return service.complete_letzshop_api(
+    result = service.complete_letzshop_api(
         vendor_id=current_user.token_vendor_id,
         api_key=request.api_key,
         shop_slug=request.shop_slug,
         letzshop_vendor_id=request.vendor_id,
     )
+    db.commit()  # Commit at API level for transaction control
+    return result
 
 
 # =============================================================================
@@ -181,7 +185,7 @@ def save_product_import_config(
     At least one CSV URL must be provided.
     """
     service = OnboardingService(db)
-    return service.complete_product_import(
+    result = service.complete_product_import(
         vendor_id=current_user.token_vendor_id,
         csv_url_fr=request.csv_url_fr,
         csv_url_en=request.csv_url_en,
@@ -190,6 +194,8 @@ def save_product_import_config(
         delivery_method=request.delivery_method,
         preorder_days=request.preorder_days,
     )
+    db.commit()  # Commit at API level for transaction control
+    return result
 
 
 # =============================================================================
@@ -209,12 +215,14 @@ def trigger_order_sync(
     Creates a background job that imports orders from Letzshop.
     """
     service = OnboardingService(db)
-    return service.trigger_order_sync(
+    result = service.trigger_order_sync(
         vendor_id=current_user.token_vendor_id,
         user_id=current_user.id,
         days_back=request.days_back,
         include_products=request.include_products,
     )
+    db.commit()  # Commit at API level for transaction control
+    return result
 
 
 @router.get(
@@ -251,7 +259,9 @@ def complete_order_sync(
     This also marks the entire onboarding as complete.
     """
     service = OnboardingService(db)
-    return service.complete_order_sync(
+    result = service.complete_order_sync(
         vendor_id=current_user.token_vendor_id,
         job_id=request.job_id,
     )
+    db.commit()  # Commit at API level for transaction control
+    return result