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_voice_profiles.py

@@ -0,0 +1,28 @@
+"""Tests for voice profile language resolution."""
+
+from __future__ import annotations
+
+from abogen.domain.enums import Language
+from abogen.voice_profiles import resolve_profile_language
+
+
+class TestResolveProfileLanguage:
+
+    def test_iso_code(self) -> None:
+        assert resolve_profile_language({"language": "en-US"}) == Language.EN_US
+        assert resolve_profile_language({"language": "es"}) == Language.ES
+
+    def test_enum_value(self) -> None:
+        assert resolve_profile_language({"language": Language.ZH}) == Language.ZH
+
+    def test_legacy_kokoro_letter(self) -> None:
+        assert resolve_profile_language({"language": "a"}) == Language.EN_US
+        assert resolve_profile_language({"language": "e"}) == Language.ES
+        assert resolve_profile_language({"language": "z"}) == Language.ZH
+
+    def test_missing_or_unparseable_falls_back(self) -> None:
+        assert resolve_profile_language({}) == Language.EN_US
+        assert resolve_profile_language({"language": ""}) == Language.EN_US
+        assert resolve_profile_language({"language": "xx"}) == Language.EN_US
+        assert resolve_profile_language(None) == Language.EN_US
+        assert resolve_profile_language([]) == Language.EN_US