feat: enhance error handling for calendar fetch failures

- Added specific handling for 404 errors in CalendarService to provide a user-friendly hint when the calendar feed is not found.
- Updated TrayContext to display the last fetch error message in the balloon tip instead of a generic message.
- Introduced a new UI text constant for the calendar fetch error title.

Author: supermem613

src/ComingUpNextTray/Services/CalendarService.cs

@@ -183,6 +183,10 @@ public async Task<IReadOnlyList<CalendarEntry>> FetchWithErrorsAsync(Uri calenda
                 {
                     hint = " Redirected location may require authentication.";
                 }
+                else if (code == 404)
+                {
+                    hint = " Calendar feed not found (404). Check the calendar URL.";
+                }
                 else if (code == 401 || code == 403)
                 {
                     hint = " Calendar feed appears to require authentication (401/403).";
@@ -237,6 +241,10 @@ public async Task<IReadOnlyList<CalendarEntry>> FetchIfChangedWithErrorsAsync(Ur
                 {
                     hint = " Redirected location may require authentication.";
                 }
+                else if (code == 404)
+                {
+                    hint = " Calendar feed not found (404). Check the calendar URL.";
+                }
                 else if (code == 401 || code == 403)
                 {
                     hint = " Calendar feed appears to require authentication (401/403).";

src/ComingUpNextTray/TrayContext.cs

@@ -391,9 +391,10 @@ private async Task RefreshAndUpdateUiAsync()
                 {
                     // Show a one-time balloon if calendar URL exists but fetch failed.
                     string url = this.app.GetCalendarUrlForUi();
-                    if (!string.IsNullOrWhiteSpace(url))
+                    string? fetchErr = this.app.GetLastFetchErrorForUi();
+                    if (!string.IsNullOrWhiteSpace(url) && !string.IsNullOrWhiteSpace(fetchErr))
                     {
-                        this.ShowErrorBalloon();
+                        this.ShowErrorBalloon(fetchErr);
                     }
                 }
             }
@@ -404,12 +405,12 @@ private async Task RefreshAndUpdateUiAsync()
         }
 
         [System.Diagnostics.CodeAnalysis.SuppressMessage("Globalization", "CA1303:Do not pass literals as localized parameters", Justification = "Using centralized UiText constants; localization pending.")]
-        private void ShowErrorBalloon()
+        private void ShowErrorBalloon(string message)
         {
             try
             {
-                this.notifyIcon.BalloonTipTitle = UiText.ConfigErrorTitle;
-                this.notifyIcon.BalloonTipText = UiText.ConfigErrorMessage;
+                this.notifyIcon.BalloonTipTitle = UiText.FetchErrorTitle;
+                this.notifyIcon.BalloonTipText = message;
                 this.notifyIcon.ShowBalloonTip(3000);
             }
             catch (System.ComponentModel.Win32Exception)

src/ComingUpNextTray/UiText.cs

@@ -65,6 +65,9 @@ internal static class UiText
         /// <summary>Body text for configuration error message box.</summary>
         internal const string ConfigErrorMessage = "Config file malformed. Renamed to config.json.invalid.";
 
+        /// <summary>Title for calendar/network fetch error balloon tips.</summary>
+        internal const string FetchErrorTitle = "Calendar Error";
+
         /// <summary>Tooltip or UI text when no calendar configured.</summary>
         internal const string NoCalendarConfigured = "No calendar URL configured";