Localise the UI into Russian and English, switchable without a restart

Strings move into Localization/Strings.resx plus a Russian satellite. XAML uses
a {l:Loc Key} markup extension that yields a binding through the localizer's
indexer rather than a resolved string, so changing the language raises
PropertyChanged for the indexer and every caption in the app re-reads at once.
Resolving strings once at load would have been simpler and would have needed an
app restart to take effect.

Three places where this is more than a string swap:

Russian has three plural forms, so counted messages are assembled from .One /
.Few / .Many keys via Localizer.Plural instead of "{0} records" with an English
plural glued on. "1 запись", "3 записи", "7 записей".

Enum captions in pickers go through LocalizedOption<T>. A value converter would
resolve the caption once and never notice a language change; the wrapper keeps
identity on the enum value so the selection survives, while the label follows
the localizer. It needed a non-generic base for DataTemplate x:DataType, since
Avalonia 12 compiles bindings by default and cannot infer one for an open
generic.

Text originating in the domain would otherwise have stayed English under a
Russian UI — the screenshots showed exactly that. AvParser.Core still knows
nothing about languages: ParseError now carries a Code and Arguments, and the UI
translates Parse.Error.{Code} with a fallback to the English message. Parser
names work the same way (Parser.{id}.Name falling back to DisplayName), which
keeps "add a parser = one registration line" true — an untranslated parser shows
its own name rather than a missing-key marker.

Both .resx files are generated from one table so a key cannot exist in one and
be missing from the other, and the tests assert that, plus no blank translations
and identical {0} placeholder sets — a translation that drops a placeholder
throws at runtime rather than merely reading oddly. A headless test switches
language on a live shell and asserts the rendered text changes without the tree
being rebuilt.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/AvParser.UI/DependencyInjection/UiServiceCollectionExtensions.cs

@@ -26,6 +26,7 @@ public static class UiServiceCollectionExtensions
 
         services.AddSingleton<ViewLocator>();
         services.AddSingleton<IThemeService, ThemeService>();
+        services.AddSingleton<ILocalizationService, LocalizationService>();
         services.AddSingleton<INavigationService, NavigationService>();
 
         services.AddSingleton<DashboardViewModel>();
@@ -39,7 +40,8 @@ public static class UiServiceCollectionExtensions
             sp.GetRequiredService<IThemeService>(),
             sp.GetRequiredService<IAppPaths>(),
             sp.GetRequiredService<LoggingLevelSwitch>(),
-            sp.GetRequiredService<IProxyPool>()
+            sp.GetRequiredService<IProxyPool>(),
+            sp.GetRequiredService<ILocalizationService>()
         ));
         services.AddSingleton<ProxiesViewModel>(static sp => new ProxiesViewModel(
             sp.GetRequiredService<IProxyPool>(),