Fix: language toggle didn't re-render page

Add :key="i18n.locale" to <router-view> so every view re-evaluates
i18n.t() when the locale changes, reliably updating all text (TH/EN).

frontend/src/App.vue

@@ -9,7 +9,9 @@
       </div>
     </nav>
     <main class="main">
-      <router-view />
+      <!-- :key forces each view to re-evaluate i18n.t() when the locale changes,
+           so switching language updates the whole page reliably. -->
+      <router-view :key="i18n.locale" />
     </main>
   </div>
 </template>