feat(gui): let the UI language be chosen manually (#179)

The interface followed the system locale with no way to override it. Settings now
carry a language picker, stored as forced_lang in gui.json, with "System" listed
first so the choice stays undoable.

rust-i18n keeps the current locale in a process-wide atomic and t! reads it on
every lookup, so switching takes effect on the next frame without a restart.

Languages are listed by their own name rather than a translated one, since that
is what a reader looking for their language recognises. A test asserts every
shipped locale has such a name, so adding a translation without naming it fails
rather than silently showing a bare language code.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

pwsp-gui/locales/app.toml

@@ -561,3 +561,25 @@ ar = "مستوى صوت الميكروفون"
 kz = "Микрофон дыбысы"
 he = "עוצמת מיקרופון"
 pt-BR = "Volume do microfone"
+
+[gui.settings.language.label]
+en = "Language"
+ru = "Язык"
+es = "Idioma"
+fr = "Langue"
+zh = "语言"
+ar = "اللغة"
+kz = "Тіл"
+he = "שפה"
+pt-BR = "Idioma"
+
+[gui.settings.language.system]
+en = "System"
+ru = "Системный"
+es = "Sistema"
+fr = "Système"
+zh = "系统"
+ar = "النظام"
+kz = "Жүйе"
+he = "מערכת"
+pt-BR = "Sistema"