Show total building block cost in multi-selection panel

Implement the REQ-UI-MULTI-SELECTION addition: the selected building
panel now shows the total placement cost of a multi-selection, counting
only player-placeable buildings (HQ and defence stations excluded).
Construction sites are charged their type's full cost.

Add a shared BuildingsConfig::findBuildingDef accessor and reuse it in
BlueprintPanel to remove the duplicated by-type lookup loops.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DZR44tA8sn4dPqDzAVXyps

src/lib/config/BuildingsConfig.h

@@ -31,4 +31,18 @@ struct BuildingDef
 struct BuildingsConfig
 {
     std::vector<BuildingDef> buildings;
+
+    // Returns the definition for the given building type, or nullptr if the
+    // type has no entry in buildings.toml.
+    const BuildingDef* findBuildingDef(BuildingType type) const
+    {
+        for (const BuildingDef& def : buildings)
+        {
+            if (def.type == type)
+            {
+                return &def;
+            }
+        }
+        return nullptr;
+    }
 };