Tests all-day event layout calculations

Adds comprehensive tests for the AllDayManager, covering various overlap scenarios.
Tests ensure correct row and column assignments for all-day events based on date ranges and overlaps.
Replaces individual event layout calculation with batch calculation for improved performance and test coverage.

test/helpers/dom-helpers.ts

@@ -0,0 +1,40 @@
+import { CalendarEvent } from '../../src/types/CalendarTypes';
+
+/**
+ * Setup mock DOM for testing
+ */
+export function setupMockDOM(): void {
+  // Create basic DOM structure for testing
+  document.body.innerHTML = `
+    <div class="swp-calendar-grid" data-current="true">
+      <div class="swp-day-header" data-date="2024-09-22"></div>
+      <div class="swp-day-header" data-date="2024-09-23"></div>
+      <div class="swp-day-header" data-date="2024-09-24"></div>
+      <div class="swp-day-header" data-date="2024-09-25"></div>
+      <div class="swp-day-header" data-date="2024-09-26"></div>
+    </div>
+    <swp-calendar-header>
+      <swp-allday-container></swp-allday-container>
+    </swp-calendar-header>
+  `;
+}
+
+/**
+ * Create mock CalendarEvent for testing
+ */
+export function createMockEvent(
+  id: string, 
+  title: string, 
+  startDate: string, 
+  endDate: string
+): CalendarEvent {
+  return {
+    id,
+    title,
+    start: new Date(startDate),
+    end: new Date(endDate),
+    type: 'work',
+    allDay: true,
+    syncStatus: 'synced'
+  };
+}