feat(app): add Chinese (Simplified) as a 7th UI language

Translate every key in Loc.Strings and register "zh" in Loc.Languages
/LanguageNames. Wire it into the Settings language picker, which is a
hand-written RadioButton list (not generated from Loc.Languages), so
add IsLangZh to MainWindowViewModel and the matching button in
MainView.axaml. Update CLAUDE.md/AGENTS.md language count and note the
non-dynamic picker to avoid the same gap next time.

src/App/Views/MainView.axaml

@@ -1511,6 +1511,9 @@
                             <RadioButton GroupName="lang" Content="Deutsch" Margin="0,0,14,4"
                                          IsChecked="{Binding IsLangDe, Mode=OneWay}"
                                          Command="{Binding SetLanguageCommand}" CommandParameter="de"/>
+                            <RadioButton GroupName="lang" Content="中文" Margin="0,0,14,4"
+                                         IsChecked="{Binding IsLangZh, Mode=OneWay}"
+                                         Command="{Binding SetLanguageCommand}" CommandParameter="zh"/>
                         </WrapPanel>
                     </StackPanel>