Extract Vue frontend into separate container and add stp_wasm crate

Move Vue app from nginx/vue/ to top-level vue/ with its own Dockerfile,
update docker-compose configs and nginx proxy to serve from the new
container, and add initial Rust WASM crate (stp_wasm). Also fix .gitignore
to exclude Rust target/ directories.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vue/src/main.js

@@ -0,0 +1,12 @@
+import { createApp } from "vue";
+import { createPinia } from "pinia";
+import App from "./App.vue";
+import router from "./router";
+import "./assets/styles.css";
+
+const app = createApp(App);
+
+app.use(router);
+app.use(createPinia());
+
+app.mount("#app");