refactor: unify Language enum across all layers

- EngineConfig.language: Language (was lang_code: str = 'a')
- Engine owns _KOKORO_LANG_MAP, engine_language(), supported_languages()
- Engine provides language_for_voice_id() for voice catalog
- Plugins/kokoro/__init__.py calls engine_language() internally
- create_pipeline(plugin_id, language=Language) — no kokoro codes
- pipeline_factory.py clean of kokoro-specific code
- Domain functions raise TypeError if non-enum passed
- WebUI api.py: _parse_language() helper at API boundary
- Voice catalog returns ISO codes (lang.value)
- Constants: LANGUAGE_DESCRIPTIONS keyed by Language enum
- All tests updated for Language enum
- 1414 tests pass

tests/test_behavioral_regression.py

@@ -928,10 +928,11 @@ class TestValueObjectsBehavioral:
 
     def test_engine_config_defaults(self) -> None:
         from abogen.tts_plugin.types import EngineConfig
+        from abogen.domain.enums import Language
 
         config = EngineConfig()
         assert config.device == "cpu"
-        assert config.lang_code == "a"
+        assert config.language == Language.EN_US
 
     def test_parameter_values_defaults(self) -> None:
         pv = ParameterValues()