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.

tests/ClaudeDo.Installer.Tests/FakeLocalizer.cs

@@ -0,0 +1,13 @@
+using ClaudeDo.Localization;
+
+namespace ClaudeDo.Installer.Tests;
+
+internal sealed class FakeLocalizer : ILocalizer
+{
+    public string this[string key] => key;
+    public string Get(string key, params object[] args) => key;
+    public string CurrentCode { get; private set; } = "en";
+    public IReadOnlyList<LanguageOption> AvailableLanguages { get; } = new[] { new LanguageOption("en", "English") };
+    public void SetLanguage(string code) => CurrentCode = code;
+    public event EventHandler? LanguageChanged;
+}