feat: add configurable visibility control for NowPlaying feature

Replaces the boolean EnableNowPlaying option with a more flexible NowPlaying configuration structure containing Enabled and AdminOnly flags. This allows three visibility modes: disabled, admin-only, and all users.

The new configuration uses nowplayingOptions struct similar to jukeboxOptions, with the following defaults:
- NowPlaying.Enabled: true (feature enabled)
- NowPlaying.AdminOnly: false (visible to all users)

The old EnableNowPlaying option is deprecated and automatically migrated to NowPlaying.Enabled with a warning message.

Frontend changes update the AppBar component to conditionally render NowPlayingPanel based on both the enabled state and the admin-only permission check. Server-side enforcement of the AdminOnly setting is added in a follow-up commit.

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

ui/src/config.js

@@ -34,6 +34,7 @@ const defaultConfig = {
   enableCoverAnimation: true,
   enableNowPlaying: true,
   playbackReportIntervalMs: 60000,
+  nowPlayingAdminOnly: false,
   devShowArtistPage: true,
   devUIShowConfig: true,
   devNewEventStream: false,

ui/src/layout/AppBar.jsx

@@ -121,8 +121,10 @@ const CustomUserMenu = ({ onClick, ...rest }) => {
   return (
     <>
       {config.devActivityPanel &&
-        permissions === 'admin' &&
-        config.enableNowPlaying && <NowPlayingPanel />}
+        config.enableNowPlaying &&
+        (!config.nowPlayingAdminOnly || permissions === 'admin') && (
+          <NowPlayingPanel />
+        )}
       {config.devActivityPanel && permissions === 'admin' && <ActivityPanel />}
       <UserMenu {...rest}>
         <PersonalMenu sidebarIsOpen={true} onClick={onClick} />

ui/src/layout/AppBar.test.jsx

@@ -39,6 +39,7 @@ describe('<AppBar />', () => {
   beforeEach(() => {
     config.devActivityPanel = true
     config.enableNowPlaying = true
+    config.nowPlayingAdminOnly = true
     store = createStore(combineReducers({ activity: activityReducer }), {
       activity: { nowPlayingCount: 0 },
     })
@@ -62,4 +63,14 @@ describe('<AppBar />', () => {
     )
     expect(screen.queryByTestId('now-playing-panel')).toBeNull()
   })
+
+  it('shows NowPlayingPanel to all users when adminOnly is false', () => {
+    config.nowPlayingAdminOnly = false
+    render(
+      <Provider store={store}>
+        <AppBar />
+      </Provider>,
+    )
+    expect(screen.getByTestId('now-playing-panel')).toBeInTheDocument()
+  })
 })