Add web-app with machine-specific builds

Implements per-machine customization for web-app deployment:
- Shared web-app source code deployed to all machines
- Machine-specific .env files and images
- Build helper script to assemble and build on target machines

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

build-helper.nix

@@ -0,0 +1,29 @@
+# Helper script for building the web-app on target machines
+# This can be imported into the machine's configuration.nix
+
+{ pkgs }:
+
+pkgs.writeShellScriptBin "build-web-app" ''
+  set -e
+
+  # Web-app sources are deployed to /var/src by krops
+  WEB_APP_DIR="/var/src/web-app"
+  ENV_FILE="/var/src/web-app-env"
+  IMAGES_DIR="/var/src/web-app-images"
+
+  cd "$WEB_APP_DIR"
+
+  # Copy machine-specific .env file
+  echo "Copying machine-specific .env file..."
+  cp "$ENV_FILE" .env
+
+  # Copy machine-specific images to public folder
+  echo "Copying machine-specific images..."
+  cp -r "$IMAGES_DIR"/* public/
+
+  # Build the web-app
+  echo "Building web-app..."
+  ${pkgs.nodejs}/bin/npm run build
+
+  echo "Build complete! Output in $WEB_APP_DIR/dist"
+''