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/contracts/test_plugin_manager_contract.py

@@ -3,6 +3,7 @@
 import pytest
 from unittest.mock import MagicMock, patch
 
+from abogen.domain.enums import Language
 from abogen.tts_plugin.plugin_manager import PluginManager, get_plugin_manager, reset_plugin_manager
 from abogen.tts_plugin.utils import Pipeline, create_pipeline
 from abogen.tts_plugin.engine import Engine, EngineSession
@@ -175,7 +176,7 @@ class TestCreatePipelineCompat:
             mock_engine = FakeEngine()
             mock_manager.create_engine.return_value = mock_engine
             
-            backend = create_pipeline("kokoro", lang_code="a", device="cpu")
+            backend = create_pipeline("kokoro", language=Language.EN_US, device="cpu")
             
             assert callable(backend)
             mock_manager.create_engine.assert_called_once()
@@ -185,7 +186,7 @@ class TestCreatePipelineCompat:
             assert call_args.kwargs["model_path"] is None
             assert isinstance(call_args.kwargs["config"], EngineConfig)
             assert call_args.kwargs["config"].device == "cpu"
-            assert call_args.kwargs["config"].lang_code == "a"
+            assert call_args.kwargs["config"].language == Language.EN_US
     
     def test_create_pipeline_raises_for_unknown_plugin(self):
         """create_pipeline raises KeyError for unknown plugins."""