Drag & drop tab reordering

Tabs are draggable within the strip: HTML5 DnD with the WebKit setData quirk
handled, an inset primary insertion line at the drop position (no layout
shift), and the dragged tab dimmed. Reordering only touches the workspaces
array order; ids, sessions, and the active tab are unaffected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/app/panels.service.ts

@@ -274,6 +274,18 @@ export class PanelsService {
     }
   }
 
+  /** Reorder a tab (drag & drop). Order is purely visual; ids stay stable. */
+  moveWorkspace(id: number, toIndex: number): void {
+    this.workspaces.update((list) => {
+      const fromIndex = list.findIndex((w) => w.id === id);
+      if (fromIndex < 0) return list;
+      const next = [...list];
+      const [moved] = next.splice(fromIndex, 1);
+      next.splice(Math.max(0, Math.min(next.length, toIndex)), 0, moved);
+      return next;
+    });
+  }
+
   cycleWorkspace(delta: 1 | -1): void {
     const list = this.workspaces();
     if (list.length < 2) return;