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/src/locale.rs

@@ -0,0 +1,58 @@
+use pwsp_lib::types::config::GuiConfig;
+
+/// Applies the UI language: the one pinned in the config, or the system's when none is.
+///
+/// Safe to call at any time — `t!` reads the locale on every lookup, so the next frame
+/// already renders in the new language.
+pub fn apply(config: &GuiConfig) {
+    let locale = config.forced_lang.clone().unwrap_or_else(system_locale);
+    rust_i18n::set_locale(&locale);
+}
+
+fn system_locale() -> String {
+    sys_locale::get_locale().unwrap_or_else(|| String::from("en-US"))
+}
+
+/// Locales shipped under `locales/`, in a stable order for the menu.
+pub fn available() -> Vec<String> {
+    let mut locales: Vec<String> = rust_i18n::available_locales!()
+        .into_iter()
+        .map(|locale| locale.to_string())
+        .collect();
+    locales.sort_unstable();
+    locales
+}
+
+/// How a language names itself, which is how it should appear in a language picker
+/// regardless of the language the rest of the UI is in. An unnamed locale falls back to
+/// its code, so a newly shipped translation is still selectable.
+pub fn display_name(code: &str) -> &str {
+    match code {
+        "en" => "English",
+        "ru" => "Русский",
+        "es" => "Español",
+        "fr" => "Français",
+        "zh" => "中文",
+        "ar" => "العربية",
+        "kz" => "Қазақша",
+        "he" => "עברית",
+        "pt-BR" => "Português (Brasil)",
+        other => other,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn every_shipped_locale_has_a_native_name() {
+        for code in available() {
+            assert_ne!(
+                display_name(&code),
+                code,
+                "locale {code} falls back to its code in the picker"
+            );
+        }
+    }
+}