feat(dl): gate s_lang/a_lang miss behind --best-available

Missing requested subtitle and audio languages now warn and continue when --best-available is set instead of hard-exiting. Without the flag, missing languages still produce an error and exit, matching the prior strict behavior. Audio missing-lang detection is now symmetric with subtitles.

- add find_missing_langs helper in core/utilities for reuse between s_lang and a_lang paths (skips all/best/orig sentinels)
- refactor dl.py s_lang/a_lang checks to share the helper
- add tests/lang_selection covering match primitives, helper output, and tricky langcodes corners (zh-Hans/zh-Hant/zh-CN/zh-TW/zh-HK, cmn/yue, fil/tl/tgl)
- clean up unused-var ruff F841 in tests/remote/unit/

tests/remote/unit/test_remote_service_helpers.py

@@ -6,16 +6,8 @@ from enum import Enum
 
 import pytest
 
-from unshackle.core.remote_service import (
-    _build_title,
-    _build_tracks,
-    _deserialize_audio,
-    _deserialize_subtitle,
-    _deserialize_video,
-    _enum_get,
-    _match_track,
-    _reconstruct_drm,
-)
+from unshackle.core.remote_service import (_build_title, _build_tracks, _deserialize_audio, _deserialize_subtitle,
+                                           _deserialize_video, _enum_get, _match_track, _reconstruct_drm)
 from unshackle.core.titles.episode import Episode
 from unshackle.core.titles.movie import Movie
 from unshackle.core.tracks import Audio, Subtitle, Video