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/settings/show.blade.php

@@ -0,0 +1,26 @@
+@extends('master')
+
+@section('title')Site Settings « Admin CP « @stop
+
+@section('content')
+    <h1>Site settings</h1>
+    <form action="/admin/settings" method="post" accept-charset="utf-8" class="admin-form form">
+        {{ csrf_field() }}
+        {{ method_field('PUT') }}
+        <div>
+            <label for="winter_effect_enabled">
+                <input
+                    type="checkbox"
+                    name="winter_effect_enabled"
+                    id="winter_effect_enabled"
+                    value="1"
+                    @checked(old('winter_effect_enabled', $settings?->winter_effect_enabled))
+                >
+                Show winter snow effect
+            </label>
+        </div>
+        <div>
+            <button type="submit" name="save">Save</button>
+        </div>
+    </form>
+@stop