Initial WebhookServer implementation

Add the .NET 8 solution scaffolded against PLAN.md. Three projects share
WebhookServer.Core (models, auth, execution, storage, IPC, callbacks)
and WebhookServer.Service hosts an embedded Kestrel listener plus the
named-pipe admin server. WebhookServer.Gui is a thin MVVM client over
the pipe. Includes 25 unit tests covering HMAC verification, bearer
auth, IP allowlist parsing, arg-template rendering, DPAPI round-trip,
and the encrypt-on-save config store.

Install/uninstall PowerShell scripts default to LocalSystem and accept
a domain user or gMSA via -ServiceAccount.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/WebhookServer.Service/ServicePaths.cs

@@ -0,0 +1,16 @@
+namespace WebhookServer.Service;
+
+/// <summary>
+/// Standard locations for runtime files (config + logs). Centralised so they're easy
+/// to override in tests and inspect in one place.
+/// </summary>
+public static class ServicePaths
+{
+    public static string DataRoot { get; } =
+        Environment.GetEnvironmentVariable("WEBHOOKSERVER_DATA")
+        ?? Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData), "WebhookServer");
+
+    public static string ConfigPath => Path.Combine(DataRoot, "config.json");
+    public static string LogsDir => Path.Combine(DataRoot, "logs");
+    public static string LogFileTemplate => Path.Combine(LogsDir, "webhook-.log");
+}