fix(ui): warn and block save when a list's working dir isn't a git repo

Reuses RepoScanner.IsRepo (now public) to validate WorkingDir whenever it
changes in ListSettingsModalViewModel — Browse or typing. A missing folder
or a non-repo folder shows an inline warning and gates Save via the same
CanExecute pattern the load-failure gate already uses; an empty WorkingDir
is untouched. Previously this only surfaced as a Failed task deep in
WorktreeManager on first run.

Also localizes the two hardcoded EN strings in the same file (delete
confirm, planning-sessions block) into vm.listSettings.* keys.

src/ClaudeDo.Ui/Views/Modals/ListSettingsModalView.axaml

@@ -94,6 +94,9 @@
                   <TextBox Grid.Column="0" Text="{Binding WorkingDir}" PlaceholderText="{loc:Tr modals.listSettings.workingDirectoryPlaceholder}" />
                   <Button Classes="btn" Grid.Column="1" Content="{loc:Tr modals.listSettings.browse}" Margin="8,0,0,0" Click="BrowseClicked" />
                 </Grid>
+                <TextBlock Text="{Binding WorkingDirWarning}"
+                           IsVisible="{Binding WorkingDirWarning, Converter={x:Static StringConverters.IsNotNullOrEmpty}}"
+                           Foreground="{DynamicResource StatusErrorBrush}" FontSize="12" TextWrapping="Wrap"/>
               </StackPanel>
 
               <StackPanel Spacing="4" IsVisible="{Binding !IsFolder}">