fix: Register VueFinderPlugin — prevents Object.keys crash on null store

VueFinder requires app.use(VueFinderPlugin) to provide its internal
context (i18n, features, config stores). Without plugin registration,
the store returned null during setup, causing Object.keys to throw
TypeError on undefined.

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

frontend/src/main.ts

@@ -2,6 +2,7 @@ import { createApp } from 'vue'
 import { createPinia } from 'pinia'
 import piniaPluginPersistedstate from 'pinia-plugin-persistedstate'
 
+import { VueFinderPlugin } from 'vuefinder'
 import App from './App.vue'
 import router from './router'
 import './style.css'
@@ -14,5 +15,6 @@ pinia.use(piniaPluginPersistedstate)
 
 app.use(pinia)
 app.use(router)
+app.use(VueFinderPlugin)
 
 app.mount('#app')