Implement toast notifications and enhance operations panel feedback

[Gabriel Dufresne (GabrielDuf)]

This pull request introduces a comprehensive overhaul of the notification and toast system in the application, focusing on clearer separation between per-operation notifications and general toasts, improved accessibility, and enhanced user feedback for batch operations. It also brings visual refinements for toast notifications and streamlines notification delivery logic across platforms.

Notification System Refactor and UI Improvements:

• Per-operation notifications no longer fall back to in-app banners: Notifications for individual operation results (progress, success, error) now only use native OS notifications on Windows/macOS, avoiding duplicate in-app banners since results are already visible in the operations panel. The fallback to in-app banners is now opt-in for other notification types. (AvaloniaOperationRegistry.cs, WindowsAppNotificationBridge.cs)

• New toast system with stack, auto-dismiss, and accessibility: Adds an observable collection of toasts (Toasts) managed by the view model, supporting up to five simultaneous toasts, auto-dismiss timers (with pause/resume on hover), and assertive/polite announcements for screen readers. Banners are now surfaced as toasts for consistency. (MainWindowViewModel.cs)

• Batch operation summary toast: When a batch of operations completes, a single summary toast (success or error) is shown, if enabled in settings, instead of multiple popups. (MainWindowViewModel.cs)

UI and Theming Enhancements:

• Distinct toast badge and shadow styling: Adds themed color and shadow resources for toast error badges in both light and dark modes, improving visual clarity. (Styles.Common.axaml)

Platform-specific and Settings UI:

• Notification type settings only shown on supported platforms: The notification type selection UI is now hidden on Linux, where native notifications are not supported. (NotificationsViewModel.cs)

These changes collectively modernize notification handling, reduce redundant alerts, and provide a more accessible and visually coherent user experience.