feat(shortcuts): bind tracklist.delete to the confirmation

The binding has been in the defaults and in Settings since it was
written, with nothing on the other end of it, because "remove from
library" did not exist. It does now — and Delete only *opens* the
dialog, never performs the removal, which is the only version
defensible one keystroke from a focused row.

The e2e case asserts the two things that matter and neither is the row
count: the file is still on disk, and a real scan of the real directory
does not bring the row back. It watches a control path survive the same
scan, because a guard that excluded everything would pass the negative
assertion for free — and it restores the database it spends.

backend/shortcuts/config.go

@@ -33,15 +33,16 @@ func DefaultBindings() map[string]string {
 		"app.selectAll": "Ctrl+A",
 		"app.shortcuts": "?",
 
-		// Panel-specific (track list).  There is no `tracklist.delete`:
-		// it was bound to Delete and advertised in Settings as
-		// configurable while nothing listened for it, because "remove
-		// from library" does not exist and it is not clear what it would
-		// remove — the row (which the next scan puts back unless the path
-		// is also excluded) or the file (a delete-your-music button one
-		// keystroke from a focused row).  Advertise it again when it does
-		// something.
-		"tracklist.play": "Enter",
+		// Panel-specific (track list).  `tracklist.delete` spent six
+		// phases advertised in Settings with nothing on the other end of
+		// it, because "remove from library" did not exist and it was not
+		// clear what it would remove.  It now removes the row and
+		// excludes the path from future scans, and leaves the file on
+		// disk — and the key only *opens the confirmation*, never
+		// performs the removal, which is the only version defensible one
+		// keystroke from a focused row.
+		"tracklist.play":   "Enter",
+		"tracklist.delete": "Delete",
 
 		// Panel-specific (autotag review).  These are the keys the
 		// autotag page used to bind on its own document listener, which