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/master.blade.php

@@ -80,6 +80,17 @@
         <!--scripts go here when needed-->
         @section('scripts')
             <script type="module" src="/assets/js/app.js"></script>
+
+            @if($winterEffectEnabled ?? false)
+                <script type="module" src="/assets/js/is-land.min.js"></script>
+                <script type="module" src="/assets/js/winter.js"></script>
+                <is-land on:media="(prefers-reduced-motion: no-preference)">
+                    <snow-fall
+                        count="150"
+                        style="--snow-fall-color: var(--clr-snow-fall)"
+                    ></snow-fall>
+                </is-land>
+            @endif
         @show
     </body>
 </html>