Notify post refreshing end with live region

[ai]

When user click on Refresh Posts, it will take a while (5 minutes+) to finish the process.

User using visual browser, can see the result visually by loader in navbar.

But screen reader user need to return Focus to Refresh progress to check the status.

It will be cool to play We loaded new posts by live region.

• Add live region to web/ui/navbar/progress.svelte
• Check it by real screen reader. On Mac there is built-in VoiceOver. On other platforms, Chrome has nice extension (but it is better to use some more popular screen reader if you want to take everything from this task).
• Add actual number of added posts reading from refreshStatistics