Bring back winter snow effect, gated by an admin setting

The site has flip-flopped on the snow effect every winter (add it,
remove it, repeat), meaning a PR round-trip each time. Instead, restore
the winter.js/is-land assets and gate them behind a new admin-toggleable
setting so the effect can be switched on/off from /admin/settings
without touching code.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0113BVPXDpEk9HPEqXvSG2WQ

resources/views/admin/welcome.blade.php

@@ -61,4 +61,9 @@
     <p>
         Manager <a href="/admin/passkeys">your passkeys</a>.
     </p>
+
+    <h2>Settings</h2>
+    <p>
+        Edit <a href="/admin/settings">site settings</a>.
+    </p>
 @stop