Windows: No taskbar badge notification for completed sessions

[rmarinho]

Description

On macOS/iOS, the app shows a badge count on the dock icon when sessions complete (using UIKit.UIApplication.SharedApplication.ApplicationIconBadgeNumber). On Windows, IncrementBadge() and ClearBadge() in CopilotService.cs are no-ops due to the #if MACCATALYST || IOS guard.

Current Behavior

Windows users get no visual indication in the taskbar when a session completes while the app is in the background.

Suggested Fix

Implement Windows taskbar badge/overlay icon using WinUI 3 BadgeNotification or TaskbarItemInfo overlay. This would provide equivalent UX to the macOS dock badge.

Files Involved

• Services/CopilotService.cs (lines ~1946-1967, IncrementBadge/ClearBadge methods)