PUT /panels/ carries a version and refuses a stale one with 409

The Panels dialog re-reads the list on 409. A panels.json without a
version reads as 0.

frontend/src/client/schemas.gen.ts

@@ -2933,6 +2933,11 @@ export const PanelsConfigSchema = {
             },
             type: 'array',
             title: 'Panels'
+        },
+        version: {
+            type: 'integer',
+            title: 'Version',
+            default: 0
         }
     },
     type: 'object',
@@ -2949,6 +2954,11 @@ export const PanelsPublicSchema = {
             type: 'array',
             title: 'Panels'
         },
+        version: {
+            type: 'integer',
+            title: 'Version',
+            default: 0
+        },
         frontend_host: {
             type: 'string',
             title: 'Frontend Host',

frontend/src/client/sdk.gen.ts

@@ -1946,6 +1946,11 @@ export class PanelsService {
      * A panel that disappears here takes its credential with it, so this is also
      * how a device is unpaired along with its assignment. Re-pairing one screen
      * and keeping the assignment is ``/{panel_id}/unpair`` below.
+     *
+     * The body carries the ``version`` it was based on, and a save someone else
+     * has moved past is refused with 409, as a dashboard's is. Two clients each
+     * writing back the whole list would otherwise drop each other's panels, and
+     * a dropped panel revokes its screen at once.
      * @param data The data for the request.
      * @param data.requestBody
      * @returns PanelsPublic Successful Response

frontend/src/client/types.gen.ts

@@ -994,6 +994,7 @@ export type direction = 'in' | 'out';
  */
 export type PanelsConfig = {
     panels?: Array<PanelDef>;
+    version?: number;
 };
 
 /**
@@ -1009,6 +1010,7 @@ export type PanelsConfig = {
  */
 export type PanelsPublic = {
     panels?: Array<PanelDef>;
+    version?: number;
     frontend_host?: string;
 };