feat(installer): add WPF installer/configurator project

Standalone WPF app (ClaudeDo.Installer) that handles full installation
and ongoing configuration of ClaudeDo. Two modes: wizard for first run,
tabbed settings panel for subsequent launches. Page-based extensibility
via IInstallerPage interface — adding new config sections requires only
one new class.

Install pipeline: dotnet publish, deploy binaries, write configs, init
DB (via SchemaInitializer from ClaudeDo.Data), register Windows Service,
create shortcuts. Dark theme matching the Avalonia app (forest teal accent).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Installer/Core/IInstallStep.cs

@@ -0,0 +1,20 @@
+namespace ClaudeDo.Installer.Core;
+
+public interface IInstallStep
+{
+    string Name { get; }
+    Task<StepResult> ExecuteAsync(InstallContext ctx, IProgress<string> progress, CancellationToken ct);
+}
+
+public sealed class StepResult
+{
+    public bool Success { get; init; }
+    public string? ErrorMessage { get; init; }
+
+    public static StepResult Ok() => new() { Success = true };
+    public static StepResult Fail(string error) => new() { Success = false, ErrorMessage = error };
+}
+
+public enum StepStatus { Pending, Running, Done, Failed, Skipped }
+
+public sealed record StepProgress(string StepName, StepStatus Status, string? Message = null);