feat(web): AddToPlaylistMenu submenu wired into TrackMenu (M7 #352)

The "Add to playlist…" entry that #372 reserved as a disabled slot
is now active. Submenu lists the operator's own playlists
alphabetically; "New playlist…" toggles an inline create form that
makes the playlist + appends the track in two API calls.

TrackMenu's existing test asserts the entry is enabled and opens the
submenu instead of the previous "is disabled with tooltip" check.

web/src/lib/components/TrackMenu.svelte

@@ -17,6 +17,7 @@
   import { useQueryClient } from '@tanstack/svelte-query';
   import FlagPopover from './FlagPopover.svelte';
   import RemoveTrackPopover from './RemoveTrackPopover.svelte';
+  import AddToPlaylistMenu from './AddToPlaylistMenu.svelte';
   import TrackMenuItem from './TrackMenuItem.svelte';
   import TrackMenuDivider from './TrackMenuDivider.svelte';
   import { user } from '$lib/auth/store.svelte';
@@ -39,6 +40,7 @@
   let menuOpen = $state(false);
   let flagOpen = $state(false);
   let removeOpen = $state(false);
+  let addOpen = $state(false);
 
   const queryClient = useQueryClient();
 
@@ -67,6 +69,7 @@
     menuOpen = false;
     flagOpen = false;
     removeOpen = false;
+    addOpen = false;
   }
 
   function onPlayNext() {
@@ -122,7 +125,7 @@
   }
 
   function onKeydown(e: KeyboardEvent) {
-    if (!menuOpen && !flagOpen && !removeOpen) return;
+    if (!menuOpen && !flagOpen && !removeOpen && !addOpen) return;
     if (e.key === 'Escape') {
       e.preventDefault();
       closeAll();
@@ -130,7 +133,7 @@
   }
 </script>
 
-<svelte:window onclick={() => (menuOpen = false)} onkeydown={onKeydown} />
+<svelte:window onclick={() => { menuOpen = false; addOpen = false; }} onkeydown={onKeydown} />
 
 <div class="relative inline-block">
   <button
@@ -164,8 +167,7 @@
       <TrackMenuItem
         icon={ListMusic}
         label="Add to playlist…"
-        disabled
-        title="Coming with playlists"
+        onclick={() => { addOpen = true; menuOpen = false; }}
       />
 
       <TrackMenuDivider />
@@ -199,4 +201,8 @@
   {#if removeOpen}
     <RemoveTrackPopover {track} onClose={closeAll} />
   {/if}
+
+  {#if addOpen}
+    <AddToPlaylistMenu {track} onClose={() => (addOpen = false)} />
+  {/if}
 </div>