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/test/components/postCard.spec.js

@@ -40,6 +40,30 @@ describe('PostCard', () => {
     expect(full.text()).not.toContain('Show more')
   })
 
+  const thumbs = (n) =>
+    Array.from({ length: n }, (_, i) => ({ image_id: 100 + i, thumbnail_url: `/t${i}` }))
+
+  it('fills the rail to full width (5 cells, no overflow) for a 6-image post', () => {
+    const post = { ...BASE, description_plain: 'd', thumbnails: thumbs(6), thumbnails_more: 0 }
+    const w = mountComponent(PostCard, { props: { post }, pinia: freshPinia() })
+    // hero + 5 rail thumbnails, no "+N" tile.
+    expect(w.findAll('.fc-post-card__rail-cell')).toHaveLength(5)
+    expect(w.find('.fc-post-card__rail-more').exists()).toBe(false)
+    // The grid spans the hero width via a column count == cells shown.
+    expect(w.find('.fc-post-card__rail').attributes('style')).toContain('--fc-rail-cols: 5')
+  })
+
+  it('reserves the last cell for a "+N" overflow tile when there are more images', () => {
+    const post = { ...BASE, description_plain: 'd', thumbnails: thumbs(6), thumbnails_more: 4 }
+    const w = mountComponent(PostCard, { props: { post }, pinia: freshPinia() })
+    // 4 thumbnails + 1 overflow tile == 5 columns; tile counts every hidden image.
+    expect(w.findAll('.fc-post-card__rail-cell')).toHaveLength(4)
+    const more = w.find('.fc-post-card__rail-more')
+    expect(more.exists()).toBe(true)
+    expect(more.text()).toBe('+5')
+    expect(w.find('.fc-post-card__rail').attributes('style')).toContain('--fc-rail-cols: 5')
+  })
+
   it('clicking an image opens the post-scoped modal (never expands the card)', async () => {
     const pinia = freshPinia()
     const modal = useModalStore()