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/ViewModels/DashboardViewModel.cs

@@ -27,7 +27,7 @@ public sealed class DashboardViewModel : PageViewModel
 
         _services = services ?? throw new ArgumentNullException(nameof(services));
 
-        Parsers = catalog.Parsers;
+        Parsers = [.. catalog.Parsers.Select(parser => new ParserViewModel(parser))];
         DataDirectory = paths.DataDirectory;
 
         GoToParseCommand = ReactiveCommand.Create(() => Navigate<ParseViewModel>());
@@ -35,13 +35,13 @@ public sealed class DashboardViewModel : PageViewModel
     }
 
     /// <inheritdoc />
-    public override string Title => "Dashboard";
+    public override string TitleKey => "Page.Dashboard";
 
     /// <inheritdoc />
     public override string IconKey => "IconHome";
 
     /// <summary>Registered parsers, shown as cards.</summary>
-    public IReadOnlyList<ITextParser> Parsers { get; }
+    public IReadOnlyList<ParserViewModel> Parsers { get; }
 
     /// <summary>Where settings and logs are written.</summary>
     public string DataDirectory { get; }