fix: address review comments for marketplace_path setting

- Fix Data Model Inconsistency: Change backend default from
  'marketplaces/default.json' to None. Now null means 'load all skills
  without filtering', and any string value means use that marketplace.

- Add Alembic migration: Create migration 099 to add marketplace_path
  column to user_settings table for enterprise deployments.

- Simplify frontend handling: Use parseMarketplacePath() utility to
  consistently convert empty string to null. Remove confusing
  DEFAULT_MARKETPLACE_PATH constant, use MARKETPLACE_PATH_PLACEHOLDER
  for UI hint only.

- Add validation: Implement isValidMarketplacePath() to validate
  marketplace path formats (simple paths like 'marketplaces/default.json'
  or cross-repo paths like 'owner/repo:path/file.json'). Show validation
  errors inline and disable save button for invalid input.

- Add tests: Frontend tests for isValidMarketplacePath() and
  parseMarketplacePath(). Backend tests for marketplace_path default
  and custom value handling.

- Add i18n: Translation for marketplace path validation error message.

Co-authored-by: openhands <openhands@all-hands.dev>

openhands/storage/data_models/settings.py

@@ -55,8 +55,9 @@ class Settings(BaseModel):
     git_user_email: str | None = None
     v1_enabled: bool = True
     # Path to the marketplace JSON file for public skills loading
-    # Defaults to 'marketplaces/default.json' in the public skills repository
-    marketplace_path: str | None = 'marketplaces/default.json'
+    # None = Load all skills without marketplace filtering
+    # String value = Use that marketplace path (e.g., 'marketplaces/default.json')
+    marketplace_path: str | None = None
 
     model_config = ConfigDict(
         validate_assignment=True,