Remove share package feature

[Marc-André Moreau (mamoreau-devolutions)]

Removes the share package feature across both WinUI and Avalonia frontends.

Changes

• Removed share UI from all package pages (toolbar buttons, context menu items) in both WinUI and Avalonia
• Removed share button from package details page (WinUI)
• Retired /v2/show-package background API endpoint (now returns HTTP 410 Gone)
• Removed OnShowSharedPackage event from BackgroundApi
• Removed showPackage deep-link handling from both MainWindow implementations
• Removed SharePackageRequested event and RequestShare command from Avalonia ViewModel
• Removed SharePackage() and ShowSharedPackage_ThreadSafe() from DialogHelper_Packages
• Removed FROM_WEB_SHARE referral enum value and SharedPackage() telemetry method