feat(web): keyboard reorder on playlist tracks (Tier C10)

Drag was already wired via @neodrag/svelte; this adds the missing
keyboard path so reorder is accessible without a pointer device.

- The drag handle becomes a real <button> (focusable). Disabled
  when canDrag is false so non-owners + unavailable tracks can't
  attempt a move.
- ArrowUp / ArrowDown on the focused handle call
  onMove(row.position, row.position +/- 1). Bounds clamping
  already lives in the page-level handler (playlists/[id]/+page.svelte:39),
  so out-of-range targets resolve as no-ops.
- Matches QueueTrackRow's pattern: aria-label "Reorder track …",
  aria-keyshortcuts, swallow Space/Enter to avoid scrolling.

Test updates: drag-handle assertions retargeted to /reorder track/
+ two new ArrowUp/ArrowDown handler tests.

web/src/lib/components/PlaylistTrackRow.svelte

@@ -41,6 +41,21 @@
     onMove?.(row.position, row.position + delta);
   }
 
+  function handleHandleKeydown(e: KeyboardEvent) {
+    if (e.key === 'ArrowUp') {
+      e.preventDefault();
+      onMove?.(row.position, row.position - 1);
+    } else if (e.key === 'ArrowDown') {
+      e.preventDefault();
+      onMove?.(row.position, row.position + 1);
+    } else if (e.key === ' ' || e.key === 'Enter') {
+      // Reorder activates via arrow keys, not Space/Enter. Prevent
+      // default so Space doesn't scroll the list and Enter doesn't
+      // fire the button's no-op click.
+      e.preventDefault();
+    }
+  }
+
   function format(sec: number): string {
     const m = Math.floor(sec / 60);
     const s = sec % 60;
@@ -74,12 +89,18 @@
   }}
 >
   {#if isOwner}
-    <span
-      class="flex-shrink-0 cursor-grab text-text-muted active:cursor-grabbing"
-      aria-label="Drag handle"
+    <button
+      type="button"
+      disabled={!canDrag}
+      aria-label="Reorder track (drag or use arrow keys)"
+      aria-keyshortcuts="ArrowUp ArrowDown"
+      onkeydown={handleHandleKeydown}
+      class="flex-shrink-0 cursor-grab text-text-muted hover:text-text-primary
+             active:cursor-grabbing focus-visible:outline focus-visible:outline-2
+             focus-visible:outline-accent disabled:cursor-not-allowed disabled:opacity-50"
     >
       <GripVertical size={14} strokeWidth={1} />
-    </span>
+    </button>
   {/if}
 
   <button