Add flake.nix with Nix derivations for website and PDF; update workflow to use nix build

.forgejo/workflows/deploy.yml

@@ -12,10 +12,18 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Build
+      - name: Build website with Nix
+        run: nix build .#website --no-write-lock-file
+
+      - name: Build speaker profile PDF
+        run: nix build .#speaker-profile --no-write-lock-file
+
+      - name: Copy build artifacts
         run: |
-          npm ci
-          npm run build
+          mkdir -p ./dist
+          cp -r result/* ./dist/
+          mkdir -p ./public/downloads
+          cp result-2/* ./public/downloads/ 2>/dev/null || true
 
       - name: Configure rclone
         run: |