chore(web/lint): clear svelte-check warnings + fix QueueDrawer test query

CI svelte-check was blocking on:

- 1 ERROR I introduced in the previous cleanup: QueueDrawer.test.ts
  passed { hidden: true } to getByLabelText, but that option only
  exists on getByRole. Fixed by switching to a direct
  document.querySelector for the aria-hidden assertion.

- 13 WARNINGS pre-existing in the codebase from M7 #352/#372/#349
  era, never surfaced because earlier CI runs failed before reaching
  type-check. Now that CI gets that far, they accumulate. Cleared:

  - FlagPopover, RemoveTrackPopover, AddToPlaylistMenu, TrackMenu:
    interactive role divs gain tabindex="-1" + onkeydown that stops
    propagation and closes on Escape (functional, not just warning-
    suppression).

  - FlagPopover state-from-props: $state(untrack(() => prop ?? default))
    for explicit initial-snapshot semantics; const isUpdate switched
    to $derived so it reacts to prop changes.

  - PlaylistTrackRow drag-div: role="listitem" added.

  - playlists/+page.svelte: autofocus replaced with bind:this + $effect.

web/src/lib/components/TrackMenu.svelte

@@ -155,9 +155,11 @@
   {#if menuOpen}
     <div
       role="menu"
+      tabindex="-1"
       class="absolute right-0 z-20 w-56 rounded-md border border-border bg-surface p-1 shadow-lg
              {direction === 'up' ? 'bottom-full mb-1' : 'top-full mt-1'}"
       onclick={(e) => e.stopPropagation()}
+      onkeydown={(e) => { e.stopPropagation(); if (e.key === 'Escape') menuOpen = false; }}
     >
       {#if !hideQueueActions}
         <TrackMenuItem icon={ListPlus} label="Play next" onclick={onPlayNext} />