feat(frontend): implement basic frontend interface

frontend/src/App.vue

@@ -0,0 +1,34 @@
+<script setup>
+import { onBeforeUnmount, onMounted } from 'vue';
+import { useRouter } from 'vue-router';
+import { toasts } from './utils/toast.js';
+
+const router = useRouter();
+
+const glyph = { success: '✓', error: '✕', info: 'i', warn: '!' };
+
+function onAuthExpired() {
+    if (router.currentRoute.value.name !== 'Login') {
+        router.push({ name: 'Login' });
+    }
+}
+
+onMounted(() => window.addEventListener('auth:expired', onAuthExpired));
+onBeforeUnmount(() => window.removeEventListener('auth:expired', onAuthExpired));
+</script>
+
+<template>
+    <router-view />
+    <div class="toasts" aria-live="polite">
+        <div
+            v-for="t in toasts"
+            :key="t.id"
+            class="toast"
+            :class="[t.type, { leaving: t.leaving }]"
+            role="status"
+        >
+            <span class="t-icon">{{ glyph[t.type] || 'i' }}</span>
+            <span class="t-msg">{{ t.message }}</span>
+        </div>
+    </div>
+</template>