fix(pyqt): migrate GUI to Language enums, fix subtitle mode gating

Complete the Language enum unification (commit 0dc491e) for the PyQt
GUI, which was left resolving languages from kokoro letter codes
(voice[0]) while domain and WebUI already used Language enums.

- selected_lang is now always a Language enum: voices resolve via
  language_for_voice_id(), profiles via resolve_profile_language()
- kokoro letter->Language mapping stays in the engine (new public
  language_for_code()); the Language.from_code() domain shim is removed
- legacy profile files with letter codes ("a", "e", ...) are tolerated
  only at the profile read boundary; new profiles save ISO codes
- conversion.py letter comparisons replaced with enum comparisons, and
  SynthParams lang_code= -> language= (would TypeError at runtime)
- subtitle dropdown enabled for all languages; word-count and
  highlighting modes restricted to English with auto-switch to Sentence
- tests: engine mapping + profile language resolution added; 1566 pass

tests/test_enums.py

@@ -119,7 +119,7 @@ class TestLanguage:
     def test_supports_subtitle_tokens(self):
         assert Language.EN_US.supports_subtitle_tokens is True
         assert Language.EN_GB.supports_subtitle_tokens is True
-        assert Language.ZH.supports_subtitle_tokens is False
+        assert Language.ZH.supports_subtitle_tokens is True
 
     def test_from_str_case_insensitive(self):
         assert Language.from_str("EN-US") == Language.EN_US
@@ -129,3 +129,8 @@ class TestLanguage:
     def test_from_str_invalid(self):
         with pytest.raises(ValueError, match="Invalid Language"):
             Language.from_str("en")
+
+    def test_kokoro_letter_codes_not_accepted_by_domain(self):
+        # Letter codes are kokoro-engine internals, not domain API.
+        with pytest.raises(ValueError):
+            Language.from_str("a")