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

app/Http/Controllers/Admin/SettingsController.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Controllers\Admin;
+
+use App\Http\Controllers\Controller;
+use App\Models\Setting;
+use Illuminate\Http\RedirectResponse;
+use Illuminate\Http\Request;
+use Illuminate\View\View;
+
+class SettingsController extends Controller
+{
+    public function show(): View
+    {
+        $settings = Setting::first();
+
+        return view('admin.settings.show', [
+            'settings' => $settings,
+        ]);
+    }
+
+    public function update(Request $request): RedirectResponse
+    {
+        $settings = Setting::firstOrNew();
+        $settings->winter_effect_enabled = $request->boolean('winter_effect_enabled');
+        $settings->save();
+
+        return redirect()->route('admin.settings.show');
+    }
+}