Add a shared NotificationCountBadge view

Extract the unread notification count capsule from the room list cell into
its own view so it can be reused elsewhere. It caps the displayed value at
99+ and comes in a medium size (used by the room list) and a small size.

AccessibilityTests/Sources/GeneratedAccessibilityTests.swift

@@ -407,6 +407,10 @@ extension AccessibilityTests {
         try await performAccessibilityAudit(named: "NoticeRoomTimelineView_Previews")
     }
 
+    func testNotificationCountBadge() async throws {
+        try await performAccessibilityAudit(named: "NotificationCountBadge_Previews")
+    }
+
     func testNotificationPermissionsScreen() async throws {
         try await performAccessibilityAudit(named: "NotificationPermissionsScreen_Previews")
     }