fix: a one-message Discord drop is labelled a Discord message, not a grouping (4390)

It stays synthetic, since teaser matching looks only at drops, but the card
no longer claims "Grouped from 1 Discord message" / "grouped by
FabledCurator" over a single message. The marker chip stays and reads
"from Discord".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/test/components/postCard.spec.js

@@ -149,13 +149,16 @@ describe('PostCard', () => {
       expect(w.find('.fc-post-card__assoc').exists()).toBe(false)
     })
 
-    it('singularises a one-message drop', () => {
+    it('does not call a one-message drop a grouping (#4390)', () => {
       const w = mountComponent(PostCard, {
         props: { post: { ...SYNTH, synthesis: { message_count: 1 } } },
         pinia: freshPinia(),
       })
-      expect(w.text()).toContain('Grouped from 1 Discord message')
-      expect(w.text()).not.toContain('1 Discord messages')
+      expect(w.text()).toContain('Discord message')
+      expect(w.text()).not.toContain('Grouped from')
+      // The marker stays — FC still wrote this post — but names the source.
+      expect(w.find('.fc-post-card__synthetic').text()).toContain('from Discord')
+      expect(w.text()).not.toContain('grouped by FabledCurator')
     })
   })