feat: building purchase particle burst effects (#57)

Adds DOM-based particle burst animations when buying buildings and
completing research projects. Blue particles for buildings, gold for
projects. Lightweight CSS animation with no external dependencies.

Refs #57 — Night of Polish, Task 1 (Visual Identity)

js/engine.js

@@ -303,6 +303,12 @@ function buyBuilding(id) {
     updateRates();
     const label = qty > 1 ? `x${qty}` : '';
     log(`Built ${def.name} ${label} (total: ${G.buildings[id]})`);
+    // Particle burst on purchase
+    const btn = document.querySelector('[onclick="buyBuilding(\'' + id + '\')"]');
+    if (btn) {
+        const rect = btn.getBoundingClientRect();
+        spawnParticles(rect.left + rect.width / 2, rect.top + rect.height / 2, '#4a9eff', 10);
+    }
     render();
 }
 
@@ -329,6 +335,12 @@ function buyProject(id) {
     }
 
     updateRates();
+    // Gold particle burst on project completion
+    const pBtn = document.querySelector('[onclick="buyProject(\'' + id + '\')"]');
+    if (pBtn) {
+        const rect = pBtn.getBoundingClientRect();
+        spawnParticles(rect.left + rect.width / 2, rect.top + rect.height / 2, '#ffd700', 16);
+    }
     render();
 }