Minor improvements to reactions code

- Use size instead of count in boosts partial to avoid extra SQL query
  on already-loaded collection
- Add else clause to reaction_path_prefix_for to raise ArgumentError
  for unknown reactable types instead of silently returning nil

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: flavorjones

app/helpers/reactions_helper.rb

@@ -3,6 +3,8 @@ def reaction_path_prefix_for(reactable)
     case reactable
     when Card then [ reactable ]
     when Comment then [ reactable.card, reactable ]
+    else
+      raise ArgumentError, "Unknown reactable type: #{reactable.class}"
     end
   end
 end

app/views/cards/display/preview/_boosts.html.erb

@@ -2,6 +2,6 @@
 <% if boosts.any? %>
   <div class="card__boosts">
     <%= image_tag "boost-color.svg", aria: { hidden: true } %>
-    <span><%= boosts.count %></span>
+    <span><%= boosts.size %></span>
   </div>
 <% end %>