filtering: hiding is an explicit per-device flag (docs/FILTERING.md)

Agreed with the owner: whether a device is on the plan is a CHECKBOX
('Hide device from plan', every kind incl. virtual), not a runtime
algorithm. The old filter survives only as the SEEDER of those flags.

- marker.hidden is the flag; hidden devices are BUILT (room LQI counts
  them — owner's decision) but rendered only in the device editor with
  'Show hidden' on, ghosted. They cast no glow and no light fill: an
  invisible device casts no visible light (owner's decision).
- seedHiddenBindings(): non-physical devices (excluded domains, Group,
  scene, bridge, myheat children, grouped lamps) in bound areas WITHOUT a
  marker. The editing client materialises them into hidden:true stub
  markers, sets settings.filter_seeded, retires settings.show_all, and
  strips fresh-hidden ids from the red-dot list. Unticking the checkbox
  keeps a hidden:false marker — the seeder never revisits a marked device,
  so the user's decision is final. New non-physical devices hide silently;
  physical ones keep the red-dot flow.
- legacy configs (no filter_seeded) keep the OLD behaviour verbatim —
  runtime filter, shared show_all, hidden-means-gone — until an editing
  client materialises them, so a read-only tablet never sees a half-state.
- 'Show all' is renamed 'Show hidden' and is LOCAL to the tab; the shared
  settings.show_all retires with the runtime filter.
- 'Remove from plan' disappears for auto/entity devices (the checkbox is
  the way); a virtual device's Delete remains a real deletion.
- docs/FILTERING.md is the source of truth for the mechanism.

Tests: seeder/seeded/legacy/lights units (146), smoke_hidden_flag with 12
assertions (68 smokes). Inventory: 146 / 51 / 43 / 68.

src/i18n/en.json

@@ -21,7 +21,7 @@
   "title.zoom_out": "Zoom out",
   "title.zoom_reset": "Reset zoom",
   "title.add_device": "Add a device to the plan",
-  "title.show_all": "Show all area devices (no filtering)",
+  "title.show_all": "Show hidden devices (ghosted, this tab only)",
   "title.markup": "Room markup: grid, lines, outlines",
   "title.configure_space": "Configure space",
   "title.add_space": "Add space",
@@ -244,7 +244,7 @@
   "opening.lock_pending": "Working…",
   "title.close_editor": "Close editor (back to view)",
   "devbar.add": "Add",
-  "devbar.show_all": "Show all",
+  "devbar.show_all": "Show hidden",
   "devbar.rules": "Icon rules",
   "space.roomcard_section": "Room card shows:",
   "space.label_temp": "Temperature",
@@ -338,5 +338,7 @@
   "btn.use": "Use",
   "confirm.delete_plan": "Delete the plan file \"{name}\" from the server? This cannot be undone.",
   "toast.plans_list_failed": "Could not list the stored plans: {err}",
-  "toast.plan_delete_failed": "Could not delete the plan: {err}"
-}
+  "toast.plan_delete_failed": "Could not delete the plan: {err}",
+  "marker.hide": "Hide device from plan",
+  "marker.hide_tip": "The device is not drawn on the plan but still counts toward the room signal. To see it: the \"Show hidden\" button in the device editor."
+}