Hold a new dashboard back until someone publishes it

A dashboard went live the moment it was created — an empty document straight to
the panels — while a new flow starts as a draft. It now works the way flows do:
published means `dashboard.json` exists, so every dashboard on every running
installation is already published and nothing needs migrating. Only the ones
created from here on start as drafts.

Mirroring FlowStore turned up a latent 500: discarding the draft of a dashboard
that had never been published unlinked its only file, and the read that followed
raised out of a 200 handler. It answers 400 now, the way a flow does.

Publishing all of them was 2N requests, because a publish has to name the
version it expects and the summaries did not carry one. They do now — and so do
the flow summaries, which had the same defect nobody had written down.

A panel had no way to hear about any of this. A publish, or a change to which
dashboards a panel carries, now puts one event on the bus and the screen
refetches what changed: no reload, so a wall display never blanks or asks for
its credential again. The subtle half is that a socket's message allowlist was
computed once at handshake — a reassigned panel would have fetched its new
document and then shown tiles that never updated.

The panels dialog logged non-superusers out. Every write in it needs a
superuser, not only the checkboxes the report mentioned, so the dialog is
read-only for everyone else. The logout itself was `main.tsx` treating 403 as a
dead session, against the contract deps.py spells out: only a 401 ends a
session, and a 403 now says so rather than silently signing someone out.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uq8mtNb97A7praJLyeEYgs

frontend/tests/widgets.spec.ts

@@ -81,9 +81,8 @@ test.beforeAll(async ({ browser }) => {
     { label: "Sat", icon: "sun", value: "9°" },
   ])
 
-  await api(page, `/dashboards/${dashboardName}`, { method: "POST" })
   const dashboard = await (
-    await api(page, `/dashboards/${dashboardName}`)
+    await api(page, `/dashboards/${dashboardName}`, { method: "POST" })
   ).json()
   dashboard.pages[0].sections[0].widgets = [
     {
@@ -179,8 +178,9 @@ test.beforeAll(async ({ browser }) => {
     data: { version: draft.version },
   })
 
-  await api(page, `/dashboards/${stackName}`, { method: "POST" })
-  const stack = await (await api(page, `/dashboards/${stackName}`)).json()
+  const stack = await (
+    await api(page, `/dashboards/${stackName}`, { method: "POST" })
+  ).json()
   stack.pages[0].sections[0].widgets = [
     {
       id: "split",