time input: make ampm derived

src/lib/TimeInput.svelte

@@ -71,7 +71,10 @@
 			minute: !value ? '' : prefixZero(value.minute.toString())
 		};
 	});
-	let ampm: ampmKey = $state('AM');
+	let ampm: ampmKey = $derived.by(() => {
+		if (!value) return 'AM';
+		return value.hour >= 12 ? 'PM' : 'AM';
+	});
 	let valid: boolean = $state(true);
 
 	let hiddenInput: HTMLInputElement;