feat: settings page with connection tab, admin notices and site health test

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Admin/AdminTab.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace LIT\MCP\Admin;
+
+/**
+ * One tab of the settings page "Einstellungen → LIT MCP".
+ */
+interface AdminTab
+{
+    public function slug(): string;
+
+    public function label(): string;
+
+    /** Registers Settings API options; runs on admin_init. */
+    public function registerSettings(): void;
+
+    /** Registers admin-post.php handlers; runs when the page is registered. */
+    public function registerActions(): void;
+
+    public function render(): void;
+}