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.json

@@ -27,6 +27,10 @@
         "compress": "./scripts/compress.sh",
         "build": "npm run lint && npm run build-css && npm run build-js && npm run compress"
     },
+    "dependencies": {
+        "@11ty/is-land": "^5.0.0",
+        "@zachleat/snow-fall": "^1.0.3"
+    },
     "allowScripts": {
         "esbuild@0.27.7": true,
         "lightningcss-cli@1.32.0": true