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/actions/serverEvents.js

@@ -1,6 +1,7 @@
 export const EVENT_SCAN_STATUS = 'scanStatus'
 export const EVENT_SERVER_START = 'serverStart'
 export const EVENT_REFRESH_RESOURCE = 'refreshResource'
+export const EVENT_NOW_PLAYING_COUNT = 'nowPlayingCount'
 
 export const processEvent = (type, data) => ({
   type,
@@ -11,6 +12,11 @@ export const scanStatusUpdate = (data) => ({
   data: data,
 })
 
+export const nowPlayingCountUpdate = (data) => ({
+  type: EVENT_NOW_PLAYING_COUNT,
+  data: data,
+})
+
 export const serverDown = () => ({
   type: EVENT_SERVER_START,
   data: {},