feat(ui): add Now Playing panel for admins (#4209)

* feat(ui): add Now Playing panel and integrate now playing count updates

Signed-off-by: Deluan <deluan@navidrome.org>

* fix: check return value in test to satisfy linter

* fix: format React code with prettier

* fix: resolve race condition in play tracker test

* fix: log error when fetching now playing data fails

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(ui): refactor Now Playing panel with new components and error handling

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(ui): adjust padding and height in Now Playing panel for improved layout

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(cache): add automatic cleanup to prevent goroutine leak on cache garbage collection

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

ui/src/eventStream.js

@@ -33,6 +33,7 @@ const startEventStream = async (dispatchFn) => {
         throttledEventHandler(dispatchFn),
       )
       newStream.addEventListener('refreshResource', eventHandler(dispatchFn))
+      newStream.addEventListener('nowPlayingCount', eventHandler(dispatchFn))
       newStream.addEventListener('keepAlive', eventHandler(dispatchFn))
       newStream.onerror = (e) => {
         // eslint-disable-next-line no-console