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_integration.py

@@ -14,6 +14,8 @@ from unittest.mock import MagicMock, patch
 
 import numpy as np
 
+from abogen.domain.enums import Language
+
 from abogen.tts_plugin.engine import Engine, EngineSession
 from abogen.tts_plugin.errors import EngineError
 from abogen.tts_plugin.plugin_manager import PluginManager, get_plugin_manager, reset_plugin_manager
@@ -328,7 +330,7 @@ class TestRegression:
         manager._loaded = True
         
         with patch("abogen.tts_plugin.utils.get_plugin_manager", return_value=manager):
-            backend = create_pipeline("mock_tts", lang_code="a", device="cpu")
+            backend = create_pipeline("mock_tts", language=Language.EN_US, device="cpu")
             
             # Old interface: pipeline(text, voice=..., speed=..., split_pattern=...)
             segments = list(backend(