Panels: per-device dashboard sets, paired by code

A panel is one screen and the ordered set of whole dashboards it shows, so a
hallway tablet and a workshop tablet carry different sets without either
dashboard knowing about the other. More than one and the device draws a rail
to switch between them — the same rail the editor puts on screen, because the
wall has it and it takes room off the canvas.

A screen has no keyboard, so it pairs rather than logs in: it shows a
six-character code, somebody approves it against a panel from the dashboards
overview, and the credential that mints reaches that panel's published
dashboards and the message endpoints its widgets speak, and nothing else.
Deleting the panel revokes it.

Closes the per-device view and the kiosk credential; supersedes the
multi-page/multi-section UI, since a page is now a dashboard of its own.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AHpLJHozysQXjsxAyU1WHj

backend/tests/conftest.py

@@ -19,6 +19,7 @@ def flow_data(tmp_path_factory: pytest.TempPathFactory) -> Generator[None, None,
     root = tmp_path_factory.mktemp("flow-data")
     settings.FLOWS_DIR = root / "flows"
     settings.SECRETS_FILE = root / "secrets.enc"
+    settings.PANELS_FILE = root / "panels.json"
     yield