feat(installer): add SystemCheckPage with blocking-error gating

Adds a new wizard page (positioned right after Welcome, FreshInstall only)
that auto-runs EnvironmentCheckService on entry and shows one row per check
with status icon, localized title/message, and hint+help-link on
failure/unknown. A "Recheck" button re-runs it, guarded against re-entrancy.

IInstallerPage gets a BlocksNavigation default member; WizardViewModel's
Next button now binds to CanGoNext, which the current page can veto (used
here while a check run is in flight or a blocking Error+Failed result is
present — Warnings and Unknown results never block). The summary line names
the blocking checks so a disabled Next is self-explanatory.

Wires up DI for the check pipeline (IProcessRunner, IPortOwnerResolver,
per-run ClaudeCliLookup) and adds the checks.* / installer.systemCheck.*
locale keys in en.json + de.json.

Visual appearance is NOT verified — needs a manual pass in the running
installer.

src/ClaudeDo.Installer/Views/WizardViewModel.cs

@@ -1,3 +1,4 @@
+using System.ComponentModel;
 using System.Linq;
 using System.Windows;
 using ClaudeDo.Installer.Core;
@@ -32,10 +33,12 @@ public partial class WizardViewModel : ObservableObject
     [NotifyPropertyChangedFor(nameof(IsLastPage))]
     [NotifyPropertyChangedFor(nameof(NextButtonText))]
     [NotifyPropertyChangedFor(nameof(CurrentPage))]
+    [NotifyPropertyChangedFor(nameof(CanGoNext))]
     private int _currentPageIndex;
 
     public IInstallerPage CurrentPage => Pages[CurrentPageIndex];
     public bool CanGoBack => CurrentPageIndex > 0;
+    public bool CanGoNext => !CurrentPage.BlocksNavigation;
     public bool IsLastPage => CurrentPageIndex == Pages.Count - 1;
     public string NextButtonText => IsLastPage
         ? (_localizer["installer.nav.install"])
@@ -58,6 +61,14 @@ public partial class WizardViewModel : ObservableObject
                           || p is InstallPageViewModel).ToList()
             : all;
 
+        // A page's blocking state (e.g. a running/failed system check) can change while it's
+        // displayed; re-evaluate CanGoNext whenever any page raises a property change.
+        foreach (var page in Pages)
+        {
+            if (page is INotifyPropertyChanged notifying)
+                notifying.PropertyChanged += (_, _) => OnPropertyChanged(nameof(CanGoNext));
+        }
+
         if (Pages.Count > 0)
             _ = InitAsync();
     }

src/ClaudeDo.Installer/Views/WizardWindow.xaml

@@ -100,6 +100,7 @@
 
                 <Button Grid.Column="2" Content="{Binding NextButtonText}"
                         Command="{Binding GoNextCommand}"
+                        IsEnabled="{Binding CanGoNext}"
                         Style="{StaticResource AccentButton}"
                         MinWidth="100"/>
             </Grid>