i18n: 5 languages (FR/EN/ZH/TH/AR), auto-detect from Windows + Settings picker

Localization infrastructure
---------------------------
PSLauncher.Core/Localization/Strings.cs is a static class exposing each
UI string as a static property. T(fr,en,zh,th,ar) helper switches by the
current language code. ~50 keys cover the visible top-level strings:
top bar, body, status badges, action buttons, "..." menu items, license
badge texts, Settings section headers, Onboarding dialog, Update dialog.

Bindings via x:Static in XAML:
  xmlns:loc="clr-namespace:PSLauncher.Core.Localization;assembly=PSLauncher.Core"
  Content="{x:Static loc:Strings.ActionLaunch}"

Auto-detection
--------------
LocalConfig gains a Language field defaulting to "auto". Strings.Init()
called at App.xaml.cs OnStartup before any UI:
- "auto" (or unknown code) → reads CultureInfo.CurrentUICulture
  .TwoLetterISOLanguageName, picks the matching supported language,
  falls back to English.
- explicit code → forced.
The chosen culture is then propagated to CurrentCulture / CurrentUICulture
so date/number formats follow.

Settings picker
---------------
SettingsDialog gets a top section "LANGUE" with a ComboBox bound to
SettingsViewModel.AvailableLanguages (Auto / FR / EN / ZH / TH / AR).
On Save, if the language code changed, prompt the user to confirm
restart, spawn `cmd /c timeout 1 & start PSLauncher.exe` and Shutdown
the current process — the new instance picks up the language at
bootstrap.

RTL for Arabic
--------------
Strings.IsRightToLeft is true when lang=="ar". App.xaml.cs sets
window.FlowDirection = RightToLeft on the MainWindow — WPF mirrors the
layout (icons on right, text aligned right).

Translations
------------
Done as best-effort by the assistant. The product wordmark "PROSERVE"
stays untranslated (brand). Logs and debug-level messages remain in
French in code — only user-visible UI is localized. Operator can
refine translations by editing Strings.cs and rebuilding.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/PSLauncher.Models/LocalConfig.cs

@@ -6,6 +6,13 @@ public sealed class LocalConfig
     public string ServerBaseUrl { get; set; } = "https://example.com/PS_Launcher/api";
     public string InstallRoot { get; set; } = string.Empty;
     public string? LastLaunchedVersion { get; set; }
+
+    /// <summary>
+    /// Code de langue de l'UI : "auto" (suit la langue Windows), "fr", "en",
+    /// "zh", "th" ou "ar". Tout changement nécessite un redémarrage du launcher.
+    /// </summary>
+    public string Language { get; set; } = "auto";
+
     public LicenseConfig License { get; set; } = new();
 }