Align landlord profile validation (#47)

frontend/src/app/services/phone-number.ts

@@ -0,0 +1,20 @@
+export function isValidPhoneNumber(value: string): boolean {
+	let compact = '';
+	for (const character of value.trim()) {
+		if (!' \t\n\r()./-'.includes(character)) compact += character;
+	}
+	if (!compact) return true;
+	if (compact.length === 10 && compact[0] === '0' && digits(compact, 0)) return true;
+	if (compact.startsWith('00')) compact = `+${compact.slice(2)}`;
+	return (
+		compact.startsWith('+') && compact.length >= 9 && compact.length <= 16 && digits(compact, 1)
+	);
+}
+
+function digits(value: string, start: number): boolean {
+	for (let index = start; index < value.length; index += 1) {
+		const character = value.charCodeAt(index);
+		if (character < 48 || character > 57) return false;
+	}
+	return true;
+}