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

backend/tests/flow/test_dashboards.py

@@ -87,6 +87,38 @@ def test_discarding_leaves_what_is_published(store: DashboardStore):
     assert not store.has_draft("house")
 
 
+def test_a_new_dashboard_is_a_draft_until_it_is_published(store: DashboardStore):
+    """Creating one does not put it on a wall; publishing is what does."""
+    created = store.write_draft(default_dashboard("house"), 0)
+
+    assert not store.is_published("house")
+    with pytest.raises(DashboardNotFound):
+        store.read("house")
+    assert store.read("house", draft=True).title == created.title
+    # Listed all the same, so the editor can find what it just made.
+    assert [(d.name, d.has_draft, d.version) for d in store.list()] == [
+        ("house", True, created.version)
+    ]
+
+    store.publish("house", created.version)
+
+    assert store.is_published("house")
+    assert store.read("house").title == created.title
+
+
+def test_an_unpublished_dashboard_can_be_renamed_and_deleted(store: DashboardStore):
+    """And renaming it does not put it on a wall either."""
+    store.write_draft(default_dashboard("house"), 0)
+
+    store.rename("house", "home")
+
+    assert not store.exists("house")
+    assert store.has_draft("home") and not store.is_published("home")
+
+    store.delete("home")
+    assert not store.exists("home")
+
+
 def test_deleting_and_renaming(store: DashboardStore):
     store.write(default_dashboard("house"))