Add default audio/subtitle language config and in-TUI settings editor

* New audio_lang / subtitle_lang fields in config (file, CLI flags, env
  vars). Forwarded to mpv's alang/slang as a comma-separated ISO 639
  priority list, e.g. "jpn,eng".
* New Settings screen (press `s` in the main view) for editing server,
  user, password, and language preferences without leaving the app.
  Credential changes trigger re-authentication against the new endpoint;
  on success the libraries/resume list is refreshed. Everything is
  persisted back to the config file the app loaded from.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

config.toml.example

@@ -1,3 +1,8 @@
 server = "http://emby.home.local"
 user = "admin"
 password = "your-password-here"
+
+# Optional default audio/subtitle language preference. Comma-separated ISO 639
+# codes in priority order; forwarded to mpv's `alang` / `slang`.
+# audio_lang = "jpn,eng"
+# subtitle_lang = "eng"