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.

vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'jsdom',
+    setupFiles: ['./test/setup.ts'],
+    globals: true,
+  },
+});