rebrand: fork Gatus as PANDUS with custom UI identity

New indigo/purple shield+pulse logo, theme colors, localStorage
key prefixes, manifest metadata. Footer and Social link retain
attribution to upstream Gatus repo per Apache 2.0.

web/app/public/manifest.json

@@ -1,8 +1,8 @@
 {
-  "id": "gatus",
-  "name": "Gatus",
-  "short_name": "Gatus",
-  "description": "Gatus is an advanced automated status page that lets you monitor your applications and configure alerts to notify you if there's an issue",
+  "id": "pandus",
+  "name": "PANDUS",
+  "short_name": "PANDUS",
+  "description": "PANDUS is a status page and health dashboard for monitoring your applications",
   "lang": "en",
   "scope": "/",
   "start_url": "/",