Modernize notification system: fix icons, dismiss lifecycle, and code quality

Replace multi-byte emoji icons with single-cell-width Unicode characters
to prevent rendering corruption in the character buffer. Hook notification
dismissal into window OnClosing event so title bar [X], Close button,
Escape key, timeout, and programmatic close all properly clean up state.
Add re-entrancy guard to prevent infinite recursion between dismiss and
close. Extract magic numbers to ControlDefaults constants.

Author: nickprotop

SharpConsoleUI/Configuration/ControlDefaults.cs

@@ -284,5 +284,21 @@ public static class ControlDefaults
 		/// Extra padding for dropdown item width calculation including shortcut spacing (default: 10)
 		/// </summary>
 		public const int MenuItemDropdownPadding = 10;
+
+		// Notification defaults
+		/// <summary>
+		/// Horizontal padding added to notification window width beyond message length (default: 8)
+		/// </summary>
+		public const int NotificationHorizontalPadding = 8;
+
+		/// <summary>
+		/// Vertical padding added to notification window height beyond message line count (default: 5)
+		/// </summary>
+		public const int NotificationVerticalPadding = 5;
+
+		/// <summary>
+		/// Default auto-dismiss timeout for notifications in milliseconds (default: 5000)
+		/// </summary>
+		public const int NotificationDefaultTimeoutMs = 5000;
 	}
 }

SharpConsoleUI/Core/NotificationSeverity.cs

@@ -29,43 +29,32 @@ public enum NotificationSeverityEnum
 
 	/// <summary>
 	/// Represents a notification severity level with associated visual properties such as colors and icons.
+	/// Uses single-cell-width Unicode characters for reliable console rendering.
 	/// </summary>
 	public class NotificationSeverity
 	{
-		/// <summary>
-		/// Represents a danger/error notification severity with a red visual theme.
-		/// </summary>
+		/// <summary>Danger/error severity with red theme. Icon: ✘ (U+2718).</summary>
 		public static readonly NotificationSeverity Danger = new NotificationSeverity(
-			"Error", "❌", NotificationSeverityEnum.Danger);
+			"Error", "\u2718", NotificationSeverityEnum.Danger);
 
-		/// <summary>
-		/// Represents an informational notification severity with a blue visual theme.
-		/// </summary>
+		/// <summary>Informational severity with blue theme. Icon: ● (U+25CF).</summary>
 		public static readonly NotificationSeverity Info = new NotificationSeverity(
-			"Info", "ℹ️", NotificationSeverityEnum.Info);
+			"Info", "\u25cf", NotificationSeverityEnum.Info);
 
-		/// <summary>
-		/// Represents a generic notification with no specific severity.
-		/// </summary>
+		/// <summary>Generic notification with no specific severity.</summary>
 		public static readonly NotificationSeverity None = new NotificationSeverity(
 			"Notification", "", NotificationSeverityEnum.None);
 
-		/// <summary>
-		/// Represents a success notification severity with a green visual theme.
-		/// </summary>
+		/// <summary>Success severity with green theme. Icon: ✔ (U+2714).</summary>
 		public static readonly NotificationSeverity Success = new NotificationSeverity(
-			"Success", "✔️", NotificationSeverityEnum.Success);
+			"Success", "\u2714", NotificationSeverityEnum.Success);
 
-		/// <summary>
-		/// Represents a warning notification severity with a yellow visual theme.
-		/// </summary>
+		/// <summary>Warning severity with yellow theme. Icon: ▲ (U+25B2).</summary>
 		public static readonly NotificationSeverity Warning = new NotificationSeverity(
-			"Warning", "⚠️", NotificationSeverityEnum.Warning);
+			"Warning", "\u25b2", NotificationSeverityEnum.Warning);
 
-		/// <summary>
-		/// Gets the severity level enum value.
-		/// </summary>
-		public NotificationSeverityEnum Severity;
+		/// <summary>Gets the severity level enum value.</summary>
+		public NotificationSeverityEnum Severity { get; }
 
 		private NotificationSeverity(string name, string icon, NotificationSeverityEnum severity)
 		{
@@ -74,21 +63,15 @@ private NotificationSeverity(string name, string icon, NotificationSeverityEnum
 			Severity = severity;
 		}
 
-		/// <summary>
-		/// Gets the icon associated with this severity level.
-		/// </summary>
+		/// <summary>Gets the icon character for this severity level.</summary>
 		public string Icon { get; }
 
-		/// <summary>
-		/// Gets the display name for this severity level.
-		/// </summary>
+		/// <summary>Gets the display name for this severity level.</summary>
 		public string? Name { get; }
 
 		/// <summary>
 		/// Gets a <see cref="NotificationSeverity"/> instance from a severity enum value.
 		/// </summary>
-		/// <param name="severity">The severity enum value.</param>
-		/// <returns>The corresponding <see cref="NotificationSeverity"/> instance.</returns>
 		/// <exception cref="NotImplementedException">Thrown if an unknown severity value is provided.</exception>
 		public static NotificationSeverity FromSeverity(NotificationSeverityEnum severity)
 		{
@@ -103,83 +86,33 @@ public static NotificationSeverity FromSeverity(NotificationSeverityEnum severit
 			};
 		}
 
-		/// <summary>
-		/// Gets the active border foreground color for this severity level.
-		/// </summary>
-		/// <param name="consoleWindowSystem">The console window system to get theme colors from.</param>
-		/// <returns>The foreground color for active window borders.</returns>
+		/// <summary>Gets the active border foreground color (always white).</summary>
 		public Color ActiveBorderForegroundColor(ConsoleWindowSystem consoleWindowSystem)
 		{
-			return Severity switch
-			{
-				NotificationSeverityEnum.Danger => Color.White,
-				NotificationSeverityEnum.Info => Color.White,
-				NotificationSeverityEnum.None => Color.White,
-				NotificationSeverityEnum.Success => Color.White,
-				NotificationSeverityEnum.Warning => Color.White,
-				_ => throw new NotImplementedException()
-			};
+			return Color.White;
 		}
 
-		/// <summary>
-		/// Gets the active title foreground color for this severity level.
-		/// </summary>
-		/// <param name="consoleWindowSystem">The console window system to get theme colors from.</param>
-		/// <returns>The foreground color for active window titles.</returns>
+		/// <summary>Gets the active title foreground color (always white).</summary>
 		public Color ActiveTitleForegroundColor(ConsoleWindowSystem consoleWindowSystem)
 		{
-			return Severity switch
-			{
-				NotificationSeverityEnum.Danger => Color.White,
-				NotificationSeverityEnum.Info => Color.White,
-				NotificationSeverityEnum.None => Color.White,
-				NotificationSeverityEnum.Success => Color.White,
-				NotificationSeverityEnum.Warning => Color.White,
-				_ => throw new NotImplementedException()
-			};
+			return Color.White;
 		}
 
-		/// <summary>
-		/// Gets the inactive border foreground color for this severity level.
-		/// </summary>
-		/// <param name="consoleWindowSystem">The console window system to get theme colors from.</param>
-		/// <returns>The foreground color for inactive window borders.</returns>
+		/// <summary>Gets the inactive border foreground color (always white).</summary>
 		public Color InactiveBorderForegroundColor(ConsoleWindowSystem consoleWindowSystem)
 		{
-			return Severity switch
-			{
-				NotificationSeverityEnum.Danger => Color.White,
-				NotificationSeverityEnum.Info => Color.White,
-				NotificationSeverityEnum.None => Color.White,
-				NotificationSeverityEnum.Success => Color.White,
-				NotificationSeverityEnum.Warning => Color.White,
-				_ => throw new NotImplementedException()
-			};
+			return Color.White;
 		}
 
-		/// <summary>
-		/// Gets the inactive title foreground color for this severity level.
-		/// </summary>
-		/// <param name="consoleWindowSystem">The console window system to get theme colors from.</param>
-		/// <returns>The foreground color for inactive window titles.</returns>
+		/// <summary>Gets the inactive title foreground color (always white).</summary>
 		public Color InactiveTitleForegroundColor(ConsoleWindowSystem consoleWindowSystem)
 		{
-			return Severity switch
-			{
-				NotificationSeverityEnum.Danger => Color.White,
-				NotificationSeverityEnum.Info => Color.White,
-				NotificationSeverityEnum.None => Color.White,
-				NotificationSeverityEnum.Success => Color.White,
-				NotificationSeverityEnum.Warning => Color.White,
-				_ => throw new NotImplementedException()
-			};
+			return Color.White;
 		}
 
 		/// <summary>
 		/// Gets the window background color for this severity level based on the current theme.
 		/// </summary>
-		/// <param name="consoleWindowSystem">The console window system to get theme colors from.</param>
-		/// <returns>The background color for notification windows of this severity.</returns>
 		public Color WindowBackgroundColor(ConsoleWindowSystem consoleWindowSystem)
 		{
 			return Severity switch