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

tests/Feature/Admin/SettingsTest.php

@@ -0,0 +1,68 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\Feature\Admin;
+
+use App\Models\Setting;
+use App\Models\User;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use PHPUnit\Framework\Attributes\Test;
+use Tests\TestCase;
+
+class SettingsTest extends TestCase
+{
+    use RefreshDatabase;
+
+    #[Test]
+    public function admin_settings_page_loads(): void
+    {
+        $user = User::factory()->make();
+
+        $response = $this->actingAs($user)
+            ->get('/admin/settings');
+        $response->assertSeeText('Site settings');
+    }
+
+    #[Test]
+    public function admin_can_enable_winter_effect(): void
+    {
+        $user = User::factory()->make();
+
+        $this->actingAs($user)
+            ->post('/admin/settings', [
+                '_method' => 'PUT',
+                'winter_effect_enabled' => '1',
+            ]);
+        $this->assertDatabaseHas('settings', ['winter_effect_enabled' => true]);
+    }
+
+    #[Test]
+    public function admin_can_disable_winter_effect(): void
+    {
+        $user = User::factory()->make();
+        Setting::factory()->create(['winter_effect_enabled' => true]);
+
+        $this->actingAs($user)
+            ->post('/admin/settings', [
+                '_method' => 'PUT',
+            ]);
+        $this->assertDatabaseHas('settings', ['winter_effect_enabled' => false]);
+    }
+
+    #[Test]
+    public function winter_effect_markup_is_hidden_when_disabled(): void
+    {
+        $response = $this->get('/');
+        $response->assertDontSee('snow-fall');
+    }
+
+    #[Test]
+    public function winter_effect_markup_is_shown_when_enabled(): void
+    {
+        Setting::factory()->create(['winter_effect_enabled' => true]);
+
+        $response = $this->get('/');
+        $response->assertSee('snow-fall', false);
+    }
+}