fix(playlist): reorder via the existing row drag gesture, not a bespoke grip

The prior reorder reused only the DragGhost but re-implemented the drag gesture with its own grip + absolute-position math, which placed the ghost several px off the cursor. Remove the grip entirely and drive reorder off the SAME row-body drag that already adds a track to a sidebar playlist: it captures the source index on start, the shared DragGhost (positioned by the Rust DragActions path) follows the cursor exactly, the drop slot is read from DragState.pointer-y against the ListView's absolute-position (no row double-count), and releasing WITHIN the list reorders while releasing on a sidebar playlist still adds. Reverted the sidebar drop suppression + removed DragState.reorder.

cc

crates/qbz-ui/ui/state.slint

@@ -968,12 +968,6 @@ export global DragState {
     in-out property <length> pointer-y;
     // The sidebar playlist id currently under the pointer (drop target).
     in-out property <string> over-playlist-id;
-    // WITHIN-LIST reorder drag (playlist custom order, issue #589): the
-    // gesture reuses this global purely for the floating ghost (DragGhost's
-    // window-coord positioning), driven from .slint only — DragActions is
-    // never called, so the Rust add-to-playlist drop path never runs.
-    // Sidebar rows skip drop-target detection while this is set.
-    in-out property <bool> reorder: false;
 }
 
 export global DragActions {