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>

tests/unit/storage/data_models/test_settings.py

@@ -127,3 +127,21 @@ def test_settings_no_pydantic_frozen_field_warning():
         assert len(frozen_warnings) == 0, (
             f'Pydantic frozen field warnings found: {[str(w.message) for w in frozen_warnings]}'
         )
+
+
+def test_settings_marketplace_path_default():
+    """Test that marketplace_path defaults to None (load all skills)."""
+    settings = Settings()
+    assert settings.marketplace_path is None
+
+
+def test_settings_marketplace_path_custom():
+    """Test that marketplace_path can be set to a custom value."""
+    settings = Settings(marketplace_path='marketplaces/custom.json')
+    assert settings.marketplace_path == 'marketplaces/custom.json'
+
+
+def test_settings_marketplace_path_none_explicit():
+    """Test that marketplace_path can be explicitly set to None."""
+    settings = Settings(marketplace_path=None)
+    assert settings.marketplace_path is None