fix: Use only dist folder (contains public assets)

- Astro build automatically copies public/ to dist/
- Remove separate public folder copy (not needed)
- Use PORT environment variable (default 80)
- Fix favicon 404 by using correct dist structure

nixpacks.toml

@@ -0,0 +1,14 @@
+# Nixpacks Configuration for Astro
+# https://nixpacks.com/docs/configuration/file
+
+[phases.setup]
+nixPkgs = ["nodejs-20_x"]
+
+[phases.install]
+cmds = ["npm ci"]
+
+[phases.build]
+cmds = ["npm run build"]
+
+[start]
+cmd = "npm run preview -- --host 0.0.0.0 --port $PORT"