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

plugins/kokoro/__init__.py

@@ -165,6 +165,7 @@ def create_engine(
     """
     try:
         KPipeline = _load_kpipeline()
+        from plugins.kokoro.engine import engine_language
 
         # Determine repo_id from model_path or use default
         repo_id = "hexgrad/Kokoro-82M"
@@ -172,8 +173,9 @@ def create_engine(
             # If a specific model path is provided, use it as repo_id
             repo_id = str(model_path)
 
+        kokoro_code = engine_language(config.language)
         pipeline = KPipeline(
-            lang_code=config.lang_code,
+            lang_code=kokoro_code,
             repo_id=repo_id,
             device=config.device,
         )