feat(api): add configurable landing page at /

Allow server operators to show new players how to install the game
client when they visit the server address in a browser. The page
content (title and HTML body) is fully configurable via config.json
and can be toggled on/off. Uses Go embed for a self-contained dark-
themed HTML template with zero new dependencies.

config.example.json

@@ -215,7 +215,12 @@
     "PatchServer": "",
     "Banners": [],
     "Messages": [],
-    "Links": []
+    "Links": [],
+    "LandingPage": {
+      "Enabled": true,
+      "Title": "My Frontier Server",
+      "Content": "<p>Welcome! Download the client from our <a href=\"https://discord.gg/example\">Discord</a>.</p>"
+    }
   },
   "Channel": {
     "Enabled": true