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/RemoveTrackPopover.svelte

@@ -55,10 +55,12 @@
 
 <div
   role="dialog"
+  tabindex="-1"
   aria-modal="false"
   aria-labelledby="remove-track-popover-title"
   class="absolute right-0 z-30 mt-1 w-72 rounded-lg border border-border bg-surface p-3 shadow-xl"
   onclick={(e) => e.stopPropagation()}
+  onkeydown={(e) => { e.stopPropagation(); if (e.key === 'Escape') onClose(); }}
 >
   <h4 id="remove-track-popover-title" class="text-sm font-medium text-text-primary">
     Remove "{track.title}" from your library?