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/tests/panel.spec.ts

@@ -107,6 +107,7 @@ test.beforeAll(async ({ browser }) => {
         ...existing,
         { id: flowName, title: "Hall", dashboards: [first, second] },
       ],
+      version: config.version,
     },
   })
   await page.close()
@@ -116,7 +117,8 @@ test.afterAll(async ({ browser }) => {
   // Restoring the list is also what removes this spec's own panel.
   if (panels) {
     const page = await apiPage(browser)
-    await api(page, "/panels/", { method: "PUT", data: { panels } })
+    const { version } = await (await api(page, "/panels/")).json()
+    await api(page, "/panels/", { method: "PUT", data: { panels, version } })
     await page.close()
   }
   await deleteAll(browser, [
@@ -240,6 +242,7 @@ async function setTouch(page: Page, touch: boolean) {
       panels: (config.panels ?? []).map((panel: PanelDef) =>
         panel.id === flowName ? { ...panel, touch } : panel,
       ),
+      version: config.version,
     },
   })
 }