Add Makefile with preview target

`make preview` runs `pnpm build && pnpm preview` so the demo can be
served from the production bundle at http://localhost:4173/. Plus
convenience targets for dev, build, test, typecheck, lint.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Makefile

@@ -0,0 +1,23 @@
+.PHONY: preview dev build test typecheck lint
+
+# Build the production bundle then serve it locally with vite preview.
+# Prints the URL (default http://localhost:4173/) once the server is ready.
+preview:
+	pnpm build
+	pnpm preview
+
+# Dev server with HMR (http://localhost:5173/).
+dev:
+	pnpm dev
+
+build:
+	pnpm build
+
+test:
+	pnpm test
+
+typecheck:
+	pnpm typecheck
+
+lint:
+	pnpm lint