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

package-lock.json

@@ -8,6 +8,10 @@
             "name": "jbuk-frontend",
             "version": "0.0.1",
             "license": "CC0-1.0",
+            "dependencies": {
+                "@11ty/is-land": "^5.0.0",
+                "@zachleat/snow-fall": "^1.0.3"
+            },
             "devDependencies": {
                 "@eslint/js": "^10.0.1",
                 "@stylistic/eslint-plugin": "^5.1.0",
@@ -20,6 +24,16 @@
                 "stylelint-config-standard": "^40.0.0"
             }
         },
+        "node_modules/@11ty/is-land": {
+            "version": "5.0.1",
+            "resolved": "https://registry.npmjs.org/@11ty/is-land/-/is-land-5.0.1.tgz",
+            "integrity": "sha512-Rh/sLhE4vrc2JaSjeY385v2UxnDY9BhnQtitETb3SKyr0A48Q5Vn06q2AvDBHObtk9+dcFWsoZX4jhT+O9g+xQ==",
+            "license": "MIT",
+            "funding": {
+                "type": "opencollective",
+                "url": "https://opencollective.com/11ty"
+            }
+        },
         "node_modules/@babel/code-frame": {
             "version": "7.29.7",
             "resolved": "https://registry.npmjs.org/@babel/code-frame/-/code-frame-7.29.7.tgz",
@@ -1018,6 +1032,12 @@
                 "url": "https://opencollective.com/typescript-eslint"
             }
         },
+        "node_modules/@zachleat/snow-fall": {
+            "version": "1.0.3",
+            "resolved": "https://registry.npmjs.org/@zachleat/snow-fall/-/snow-fall-1.0.3.tgz",
+            "integrity": "sha512-Y9srRbmO+k31vSm+eINYRV9DRoeWGV5/hlAn9o34bLpoWo+T5945v6XGBrFzQYjhyEGB4j/4zXuTW1zTxp2Reg==",
+            "license": "MIT"
+        },
         "node_modules/acorn": {
             "version": "8.17.0",
             "resolved": "https://registry.npmjs.org/acorn/-/acorn-8.17.0.tgz",