feat: add Notifications view for persistent visibility of ADO notifications

[CompN3rd]

Problem

Toast notifications (VS Code information/warning messages) are easily missed — they auto-dismiss and leave no persistent record. Users can miss PR review requests, PR status changes, or PR comment replies if they are not looking at the screen at the right moment.

Proposed Solution

Add a dedicated Notifications view in the ADOExt activity bar or sidebar (similar to the GitHub VS Code extension's GitHub Notifications panel), which lists all received notifications with their status (read/unread) and allows users to act on them directly.

Expected behaviour

• All incoming ADO notifications (PR comments, review requests, status changes) are appended to the view as items.
• Unread items are visually distinguished (e.g. bold label or badge).
• Clicking an item opens the relevant PR / work item detail panel.
• Items can be dismissed / marked as read individually or all-at-once.
• The view persists across focus changes, so no notification is silently lost.

Motivation

The GitHub VS Code extension's Notifications view is a good reference implementation. Toast-only notifications are insufficient for a workflow where users may be focused in the editor for extended periods.

Acceptance Criteria

• A new tree view contribution (e.g. �doext.notificationsView) is registered and visible in the sidebar/activity bar.
• Notifications are populated by the existing NotificationService / handlers instead of (or in addition to) �scode.window.show*Message calls.
• Unread count badge is shown on the view container icon.
• Clicking a notification item navigates to the relevant detail panel.
• A Mark all as read / Clear action is available in the view title bar.