feat(ui): use a 24h TimePicker for prime schedule time entry

Replace the free-text time TextBox (which silently reset bad input to 07:00)
with Avalonia's TimePicker (24-hour, 5-minute steps), making invalid times
impossible. Drops the now-unused TimeSpanToHhmmConverter.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Ui/Converters/TimeSpanToHhmmConverter.cs

@@ -1,21 +0,0 @@
-using System.Globalization;
-using Avalonia.Data.Converters;
-
-namespace ClaudeDo.Ui.Converters;
-
-public sealed class TimeSpanToHhmmConverter : IValueConverter
-{
-    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture) =>
-        value is TimeSpan t ? $"{t.Hours:00}:{t.Minutes:00}" : "07:00";
-
-    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
-    {
-        if (value is not string s) return new TimeSpan(7, 0, 0);
-        var parts = s.Split(':');
-        if (parts.Length == 2 &&
-            int.TryParse(parts[0], out var h) && h is >= 0 and <= 23 &&
-            int.TryParse(parts[1], out var m) && m is >= 0 and <= 59)
-            return new TimeSpan(h, m, 0);
-        return new TimeSpan(7, 0, 0);
-    }
-}