feat(app): mount PostModal at app root next to ImageViewer — single instance driven by usePostModalStore so PostCard can open from anywhere

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/App.vue

@@ -4,6 +4,7 @@
       <RouterView />
     </AppShell>
     <ImageViewer v-if="modal.isOpen" @close="modal.close()" />
+    <PostModal />
     <AppSnackbar ref="snackbar" />
   </v-app>
 </template>
@@ -13,6 +14,7 @@ import { onMounted, ref } from 'vue'
 import AppShell from './components/AppShell.vue'
 import AppSnackbar from './components/AppSnackbar.vue'
 import ImageViewer from './components/modal/ImageViewer.vue'
+import PostModal from './components/posts/PostModal.vue'
 import { useModalStore } from './stores/modal.js'
 
 const modal = useModalStore()