feat(admin): draggable dashboard panels + business KPI redesign

- Dashboard: 16 business-focused panels (KPIs, charts, tables, feeds)
- New data queries: AOV, conversion rate, revenue by category, top spenders,
  purchase funnel, 30-day trend, geography, wallet totals
- ApexCharts: 30-day revenue line, category bar, funnel bar, geo bar,
  conversion mini-donut, revenue sparkline
- SortableJS: draggable panels within rows, layout saved to localStorage
- Theme persistence: body classes (nav-dark, header-fixed, etc.) saved to
  localStorage and restored on load, MutationObserver syncs all toggles
- Panel data-panel-id attributes for layout persistence
- All panels use existing SmartAdmin panel-icon/card styling

src/admin/views/layout.ejs

@@ -75,6 +75,10 @@
             justify-content: center;
             gap: 0.4rem;
         }
+        /* Draggable panel cursor */
+        .panel-hdr { cursor: grab; }
+        .panel-hdr:active { cursor: grabbing; }
+        .sortable-ghost { opacity: 0.4; }
     </style>
 </head>