feat: merge duplicate notifications (#2056)

Use the `ignoreDuplicates` prop from vue3-notification to ignore duplicate notifications, show a count (×N) instead when notifications are merged.

Superseeds and closes https://github.com/go-vikunja/vikunja/pull/971

---------

Co-authored-by: Dominik Pschenitschni <6173598+dpschen@users.noreply.github.com>

frontend/tests/e2e/misc/notifications.spec.ts

@@ -0,0 +1,30 @@
+import {test, expect} from '../../support/fixtures'
+
+test.describe('Duplicate Notifications', () => {
+	test('Merges duplicate notifications and shows count', async ({authenticatedPage: page}) => {
+		await page.goto('/')
+		await page.waitForLoadState('networkidle')
+
+		// Trigger the same notification twice via the Vue app using $notify directly
+		await page.evaluate(() => {
+			const app = document.getElementById('app')
+			const vueApp = (app as any).__vue_app__
+			vueApp.config.globalProperties.$notify({
+				type: 'success',
+				title: 'Test',
+				text: 'Duplicate Test',
+			})
+			vueApp.config.globalProperties.$notify({
+				type: 'success',
+				title: 'Test',
+				text: 'Duplicate Test',
+			})
+		})
+
+		// Should only show one notification with a count of ×2
+		const notification = page.locator('.global-notification .vue-notification.success')
+		await expect(notification).toHaveCount(1)
+		await expect(notification.locator('.notification-content')).toContainText('Duplicate Test')
+		await expect(notification.locator('.notification-content')).toContainText('×2')
+	})
+})