feat(post-card): absorb PostModal into PostCard with click-to-expand

PostCard and PostModal competed for the same data and rendered redundant
chrome (header twice, image grid twice, attachment list twice). The wider
PostCard layout we shipped 2026-05-27 has enough real estate to be the
canonical post surface, so collapse the two into one.

Compact (default) state is unchanged: hero + 3-cell rail + truncated
title + 3/5-line description + attachment count badge. Whole-card click
expands in place. Expanded state shows: full title, mosaic of ALL post
images via PostImageGrid (uncapped, lazy-loaded via getPostFull), full
sanitized-HTML description with paragraph wrapping, attachments as
downloadable pill links. Click the chevron in the header to collapse;
mosaic image clicks open ImageViewer scoped to the post (modalStore's
postImageIds path is preserved — only the comment changed).

Per-card local state — no global modal store. Each PostCard owns its
own expanded ref and lazy-loaded detail; collapsing a card discards
neither (so re-expand is instant after the first fetch).

Deleted: PostModal.vue, postModal.js store. Removed the App.vue mount.

frontend/src/App.vue

@@ -4,7 +4,6 @@
       <RouterView />
     </AppShell>
     <ImageViewer v-if="modal.isOpen" @close="modal.close()" />
-    <PostModal />
     <AppSnackbar ref="snackbar" />
   </v-app>
 </template>
@@ -14,7 +13,6 @@ 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()