feat(ui): badge an album drag with how many tracks it carries

Dragging an album to the queue put its cover under the cursor and said
nothing about how much that was -- an album is 1 track or 30 and the
thumbnail is the same picture either way, so the one number the drop is
about was the one thing the drag did not show. Every other drag in the
app already says it; this was the exception, because it had a picture
to show instead.

A count of 1 draws no badge: "1" over a single cover is noise, and the
absence reads clearly beside a badge that only appears above one.

The badge sits inside the cover's box rather than overhanging it,
because setDragImage snapshots the element and anything outside it
risks being clipped -- while padding the box instead would move the
cover away from the cursor.

Refs #19

frontend/src/components/cover-grid/cover-grid.ts

@@ -1480,12 +1480,16 @@ export class CoverGrid
             source: 'cover-grid',
         });
 
-        // Single album: show cover art thumbnail.
-        // Multiple albums: show track-count badge.
+        // Single album: show its cover, badged with how many tracks are
+        // on the way -- an album is 1 track or 30 and the thumbnail is
+        // the same picture either way, so the number the drop is about
+        // was the one thing this drag did not say.
+        // Multiple albums: show the track-count badge alone.
         if (isSingleAlbum && hit.album.CoverArtPath) {
             this.dragImageEl =
                 createAlbumArtDragImage(
                     this.getCoverUrl(hit.album),
+                    filePaths.length,
                 );
         } else {
             this.dragImageEl = createDragImage(

frontend/src/utils/drag-image.ts

@@ -29,11 +29,23 @@ export function createDragImage(count: number): HTMLElement {
 }
 
 /**
- * Creates a drag image showing an album cover art thumbnail.
- * Falls back to the track-count badge if the image fails to load.
+ * Creates a drag image showing an album cover art thumbnail, with a
+ * corner badge saying how many tracks are on the way.
+ *
+ * The count is not decoration. The cover says *what* is being dragged
+ * and nothing said *how much* — an album is 1 track or 30 and the
+ * thumbnail is identical either way, so the one number the drop is
+ * about was the one thing the drag did not show. Every other drag in
+ * the app says it (`createDragImage` is a count and nothing else);
+ * this one was the exception because it had a picture to show instead.
+ *
+ * A count of 1 draws no badge: "1" over a single album cover is noise,
+ * and the absence is unambiguous next to a badge that only ever
+ * appears when there is more than one.
  */
 export function createAlbumArtDragImage(
     coverUrl: string,
+    count = 1,
 ): HTMLElement {
     const size = 64;
     const wrapper = document.createElement('div');
@@ -44,6 +56,12 @@ export function createAlbumArtDragImage(
         'left: -1000px',
         'pointer-events: none',
         'z-index: 9999',
+        // The badge is positioned against this box, and the box stays
+        // exactly the cover's size: anything outside it risks being
+        // clipped out of the snapshot the browser takes, and padding
+        // it instead would move the cover away from the cursor.
+        `width: ${size}px`,
+        `height: ${size}px`,
     ].join(';');
 
     const img = document.createElement('img');
@@ -61,11 +79,44 @@ export function createAlbumArtDragImage(
     ].join(';');
 
     wrapper.appendChild(img);
+
+    if (count > 1) {
+        wrapper.appendChild(countBadge(count));
+    }
+
     document.body.appendChild(wrapper);
 
     return wrapper;
 }
 
+/** The corner badge on a multi-track drag image. */
+function countBadge(count: number): HTMLElement {
+    const badge = document.createElement('span');
+
+    badge.className = 'drag-count-badge';
+    badge.textContent = String(count);
+    badge.style.cssText = [
+        'position: absolute',
+        'top: 3px',
+        'right: 3px',
+        'min-width: 20px',
+        'height: 20px',
+        'padding: 0 5px',
+        'box-sizing: border-box',
+        'border-radius: 10px',
+        'background: #ffd43b',
+        'color: #000',
+        'font-size: 12px',
+        'font-weight: 600',
+        'font-family: inherit',
+        'line-height: 20px',
+        'text-align: center',
+        'box-shadow: 0 1px 4px rgba(0,0,0,0.5)',
+    ].join(';');
+
+    return badge;
+}
+
 /**
  * Creates a drag image styled like a queue track card showing the
  * track title and artist.  Used when dragging a single track.