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/stores/modal.js

@@ -11,9 +11,9 @@ export const useModalStore = defineStore('modal', () => {
   const error = ref(null)
 
   // Post-scoped cycle. When set, prev/next cycles within this array
-  // (used by PostModal's PostImageGrid clicks). When null, prev/next
-  // falls back to current.value.neighbors (the gallery-store-driven
-  // /api/gallery/image/<id> neighbors).
+  // (used by PostCard's expanded-mosaic PostImageGrid clicks). When
+  // null, prev/next falls back to current.value.neighbors (the
+  // gallery-store-driven /api/gallery/image/<id> neighbors).
   const postImageIds = ref(null)
   const postImageIndex = ref(0)