feat: add Cloudflare Worker to serve viewer at clean subdomain

Proxy viewer.geolibre.app to the GitHub Pages build at geolibre.app/demo,
which avoids Cloudflare's 25 MiB per-asset limit on the 32 MiB DuckDB WASM
bundle while exposing a clean subdomain. Includes a workflow to deploy the
worker on changes.

Author: giswqs

.github/workflows/deploy-viewer.yml

@@ -0,0 +1,24 @@
+name: Deploy viewer
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - "workers/viewer/**"
+      - ".github/workflows/deploy-viewer.yml"
+  workflow_dispatch:
+
+jobs:
+  deploy:
+    name: Deploy viewer proxy to Cloudflare Workers
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v6
+
+      - name: Deploy to Cloudflare Workers
+        uses: cloudflare/wrangler-action@v3
+        with:
+          apiToken: ${{ secrets.CLOUDFLARE_API_TOKEN }}
+          accountId: ${{ secrets.CLOUDFLARE_ACCOUNT_ID }}
+          workingDirectory: workers/viewer

workers/viewer/src/index.ts

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+// viewer.geolibre.app
+//
+// Serves the GeoLibre web viewer at a clean subdomain by proxying to the build
+// already published at https://geolibre.app/demo (GitHub Pages). We proxy rather
+// than re-host the files because the viewer bundles a 32 MiB DuckDB WASM asset,
+// which exceeds Cloudflare's 25 MiB per-asset limit for Workers/Pages. GitHub
+// Pages has no such limit, so it stays the origin of record.
+//
+// The viewer build uses relative asset paths, so requests map 1:1:
+//   viewer.geolibre.app/<path>?<query> -> geolibre.app/demo/<path>?<query>
+//
+// Origin redirects (e.g. trailing slash) are followed server-side so the public
+// viewer.geolibre.app URL is preserved and geolibre.app/demo is never exposed.
+
+const ORIGIN = "https://geolibre.app/demo";
+
+export default {
+  async fetch(request: Request): Promise<Response> {
+    const url = new URL(request.url);
+    const target = `${ORIGIN}${url.pathname}${url.search}`;
+    return fetch(target, request);
+  },
+};

workers/viewer/wrangler.toml

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+name = "geolibre-viewer"
+main = "src/index.ts"
+compatibility_date = "2025-03-01"
+
+# Binds the custom domain on deploy. Because geolibre.app is managed in this
+# Cloudflare account, the DNS record and TLS certificate are provisioned
+# automatically on the first deploy.
+[[routes]]
+pattern = "viewer.geolibre.app"
+custom_domain = true