feat: add drag-to-rearrange and resize handles to dashboard widgets

Edit mode now supports:
- Drag handle (grip icon, top-left) to rearrange widgets via HTML5 DnD
  (drops swap widget positions, persists via API)
- Resize handle (corner icon, bottom-right) with mousedown→mousemove→mouseup
  tracking to change widget width/height in grid units, persists via API
- Cursor feedback: grab cursor on draggable widgets, se-resize on handle
- Visual feedback: dragging widget shows 50% opacity

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/lib/api.ts

@@ -331,6 +331,15 @@ export async function createWidget(dashboardId: string, data: {
   return request<DashboardWidget>(`/dashboards/${dashboardId}/widgets`, { method: "POST", body: JSON.stringify(data) });
 }
 
+export async function updateWidget(dashboardId: string, widgetId: string, data: {
+  title?: string;
+  widget_type?: string;
+  position?: { x: number; y: number; w: number; h: number };
+  config?: Record<string, unknown>;
+}): Promise<{ data: DashboardWidget | null; error: string | null }> {
+  return request<DashboardWidget>(`/dashboards/${dashboardId}/widgets/${widgetId}`, { method: "PATCH", body: JSON.stringify(data) });
+}
+
 export async function deleteWidget(dashboardId: string, widgetId: string): Promise<{ data: { ok: boolean } | null; error: string | null }> {
   return request<{ ok: boolean }>(`/dashboards/${dashboardId}/widgets/${widgetId}`, { method: "DELETE" });
 }