feat(icon): Phosphor glyph card — hero + real-UI-size strip (T-313)

The icon card renders each glyph at a hero size plus a continuous sample
strip (10–48px) so legibility is judged at the sizes the app uses, with the
entry's optional label + description and a per-entry or card-level color.
Wired end to end: icon.show publishes on the `icon` bus, the Claude
extension injects an IconMessage, and conversation_view paints the card
(display-only, D-78). en+nl catalogs.

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

lib/builtin/claude/src/activity_cluster.dart

@@ -163,6 +163,7 @@ bool _isFoldable(ConversationItem item, FoldLevel level, Map<String, String> too
     case AssistantTextMessage():
     case ImageMessage():
     case DrawingMessage():
+    case IconMessage():
       return false;
     // Thinking folds at L2+, first-class at L1.
     case AssistantThinkingMessage():