Improves all-day event layout calculation

Refactors all-day event rendering to use a layout engine
for overlap detection and positioning, ensuring events
are placed in available rows and columns.

Removes deprecated method and adds unit tests.

test/setup.ts

@@ -0,0 +1,13 @@
+import { beforeEach } from 'vitest';
+
+// Global test setup
+beforeEach(() => {
+  // Clear DOM before each test
+  document.body.innerHTML = '';
+  document.head.innerHTML = '';
+  
+  // Reset any global state
+  if (typeof window !== 'undefined') {
+    // Clear any event listeners or global variables if needed
+  }
+});