refactor(settings): serialize birthday date explicitly

`savePrimaryAccountProperty` does only allow  `string | boolean`.
Incidentally, relying on `@nextcloud/axios` default serialization, `Date`s where serialized using `toISOString`.
See https://github.com/axios/axios/blob/a7d7a714498452e611943d9d8e5c2ed8b00aa6b9/lib/helpers/toFormData.js#L134

This change corrects the usage of `savePrimaryAccountProperty` and makes serialization explicit.

Signed-off-by: Oleksandr Dzhychko <hey@oleks.dev>

Author: odzhychko

apps/settings/src/components/PersonalInfo/BirthdaySection.spec.js

@@ -0,0 +1,62 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { mount } from '@vue/test-utils'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+let personalInfoParameters
+vi.mock('@nextcloud/initial-state', () => ({
+	loadState(app, key, fallback) {
+		if (app === 'settings' && key === 'personalInfoParameters' && personalInfoParameters !== undefined) {
+			return personalInfoParameters
+		}
+		if (fallback !== undefined) {
+			return fallback
+		}
+
+		console.error('Unexpected loadState call without fallback', { app, key })
+		throw new Error()
+	},
+}))
+
+const savePrimaryAccountProperty = vi.hoisted(() => vi.fn())
+vi.mock('../../service/PersonalInfo/PersonalInfoService.js', () => ({
+	savePrimaryAccountProperty,
+}))
+
+afterEach(() => {
+	personalInfoParameters = undefined
+	vi.resetModules()
+})
+
+describe('BirthdaySection', () => {
+	it('saves value', async () => {
+		personalInfoParameters = {
+			birthdate: {
+				name: 'birthdate',
+				value: null,
+			},
+		}
+		savePrimaryAccountProperty.mockReturnValue(Promise.resolve({
+			ocs: { meta: { status: 'ok' } },
+		}))
+		const BirthdaySection = await import('./BirthdaySection.vue')
+		const wrapper = mount(BirthdaySection.default, {
+			mocks: {
+				t: (_app, text) => text,
+			},
+		})
+
+		const input = wrapper.find('input')
+		await input.setValue('1987-12-01')
+
+		await expect.poll(() => savePrimaryAccountProperty.mock.calls.length).toBe(1)
+		expect(savePrimaryAccountProperty).toHaveBeenCalledWith(
+			'birthdate',
+			'1987-12-01T00:00:00.000Z',
+		)
+		expect(input.element.value).toBe('1987-12-01')
+	})
+})

apps/settings/src/components/PersonalInfo/BirthdaySection.vue

@@ -91,7 +91,7 @@ export default {
 			try {
 				const responseData = await savePrimaryAccountProperty(
 					this.birthdate.name,
-					value,
+					value.toISOString(),
 				)
 				this.handleResponse({
 					value,