feat(i18n): add ES, FR, PT, DE languages and fix live switching

Add Spanish, French, Portuguese, German translations to all UI strings.

Fix language switching in Avalonia compiled bindings: instead of firing
PropertyChanged("Item[]") on a singleton (which Avalonia ignores when the
object reference is unchanged), Loc.SwitchTo() now creates a new Loc
instance for the selected language and replaces both Loc.Instance and the
ViewModel's _loc field. OnPropertyChanged("Loc") then forces Avalonia to
re-evaluate all {Binding Loc[key]} bindings with the new instance.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App/Views/MainWindow.axaml

@@ -37,6 +37,18 @@
                     <MenuItem Header="English" ToggleType="Radio"
                               IsChecked="{Binding IsLangEn, Mode=OneWay}"
                               Command="{Binding SetLanguageCommand}" CommandParameter="en"/>
+                    <MenuItem Header="Español" ToggleType="Radio"
+                              IsChecked="{Binding IsLangEs, Mode=OneWay}"
+                              Command="{Binding SetLanguageCommand}" CommandParameter="es"/>
+                    <MenuItem Header="Français" ToggleType="Radio"
+                              IsChecked="{Binding IsLangFr, Mode=OneWay}"
+                              Command="{Binding SetLanguageCommand}" CommandParameter="fr"/>
+                    <MenuItem Header="Português" ToggleType="Radio"
+                              IsChecked="{Binding IsLangPt, Mode=OneWay}"
+                              Command="{Binding SetLanguageCommand}" CommandParameter="pt"/>
+                    <MenuItem Header="Deutsch" ToggleType="Radio"
+                              IsChecked="{Binding IsLangDe, Mode=OneWay}"
+                              Command="{Binding SetLanguageCommand}" CommandParameter="de"/>
                 </MenuItem>
                 <MenuItem Header="{Binding Loc[settings.unit.short]}">
                     <MenuItem Header="PLM" ToggleType="Radio"