fix(posts): post-card thumbnail strip spans the full hero width

The hero collage's thumbnail rail hard-capped at 3 fixed-80px cells, left-
aligned, so it never reached the edge of the (50%-width) hero. Make the rail a
CSS grid of equal columns (1fr) at a fixed height that stretches to the hero's
full width: show up to 5 thumbnails, and when a post has more images than fit,
the last cell becomes the "+N" overflow tile (count unchanged). Column count is
driven by --fc-rail-cols so the strip always reaches the hero edge regardless
of image count.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/posts/PostCard.vue

@@ -34,7 +34,10 @@
           >
             <img :src="hero.thumbnail_url" alt="hero thumbnail" loading="lazy" />
           </button>
-          <div v-if="rail.length || moreCount" class="fc-post-card__rail">
+          <div
+            v-if="rail.length || moreCount" class="fc-post-card__rail"
+            :style="{ '--fc-rail-cols': railCols }"
+          >
             <button
               v-for="t in rail" :key="t.image_id" type="button"
               class="fc-post-card__rail-cell"
@@ -117,13 +120,29 @@ const totalImages = computed(() => images.value.length + (props.post.thumbnails_
 const plainTitle = computed(() => toPlainText(props.post.post_title))
 
 const hero = computed(() => images.value[0])
-const rail = computed(() => images.value.slice(1, 4))
+// The thumbnail strip spans the hero's full width (CSS grid, equal columns),
+// rather than a fixed 3-cell cap. Show up to RAIL_MAX cells; when there are
+// more images than fit, the last cell becomes a "+N" overflow tile so the
+// count stays accurate.
+const RAIL_MAX = 5
+const serverMore = computed(() => props.post.thumbnails_more || 0)
+const afterHero = computed(() => images.value.slice(1))
+const hasOverflow = computed(
+  () => serverMore.value > 0 || afterHero.value.length > RAIL_MAX,
+)
+const rail = computed(() =>
+  hasOverflow.value
+    ? afterHero.value.slice(0, RAIL_MAX - 1)
+    : afterHero.value.slice(0, RAIL_MAX),
+)
 const visibleCount = computed(() => (images.value.length ? 1 + rail.value.length : 0))
 const moreCount = computed(() => {
-  const more = props.post.thumbnails_more || 0
-  const extraShown = Math.max(0, images.value.length - visibleCount.value)
-  return more + extraShown
+  if (!hasOverflow.value) return 0
+  return serverMore.value + Math.max(0, images.value.length - visibleCount.value)
 })
+// Columns = thumbnails shown plus the overflow tile, so the strip fills the
+// hero's full width with equal cells.
+const railCols = computed(() => rail.value.length + (moreCount.value > 0 ? 1 : 0))
 
 const sortDateIso = computed(() => props.post.post_date || props.post.downloaded_at)
 const absoluteDate = computed(() => new Date(sortDateIso.value).toLocaleString())
@@ -279,11 +298,16 @@ function formatBytes (n) {
 .fc-post-card__hero:hover img,
 .fc-post-card__rail-cell:hover img { transform: scale(1.03); filter: brightness(1.08); }
 
+/* Equal columns spanning the hero's full width — cells stretch horizontally
+   (1fr) at a fixed height, so the strip always reaches the edge of the hero
+   regardless of how many thumbnails the post has. */
 .fc-post-card__rail {
-  display: flex; gap: 6px; margin-top: 6px; flex-wrap: wrap;
+  display: grid;
+  grid-template-columns: repeat(var(--fc-rail-cols, 3), 1fr);
+  gap: 6px; margin-top: 6px;
 }
 .fc-post-card__rail-cell {
-  width: 80px; height: 80px;
+  width: 100%; height: 84px;
   overflow: hidden; border-radius: 4px;
 }
 .fc-post-card__rail-cell img {
@@ -291,7 +315,7 @@ function formatBytes (n) {
   object-fit: cover; display: block;
 }
 .fc-post-card__rail-more {
-  width: 80px; height: 80px;
+  width: 100%; height: 84px;
   display: flex; align-items: center; justify-content: center;
   border: 1px dashed rgb(var(--v-theme-on-surface-variant));
   border-radius: 4px;