Rename "Play later" to "Add to queue" in all menus

Changes the menu action label from "Play later" to "Add to queue"
for better clarity and consistency with standard music player UX.

Updated in:
- AlbumMenu.svelte
- TrackMenu.svelte
- ArtistDetailView.svelte (tracks context menu)

Relates to #21

src/lib/components/views/ArtistDetailView.svelte

@@ -1333,7 +1333,7 @@
                     Play Next
                   </button>
                   <button class="context-menu-item" onclick={() => { handlePlayAllTracksLater(); showTracksContextMenu = false; }}>
-                    Play Later
+                    Add to queue
                   </button>
                   <button class="context-menu-item" onclick={() => { handleShuffleAllTracks(); showTracksContextMenu = false; }}>
                     Shuffle