hidden devices: blue ghosts, no live-state paint

A hidden device and an unavailable one both rendered as translucent dark —
indistinguishable at a glance, and a lit hidden lamp still glowed yellow
through the ghost (owner's report). A ghost is CONFIGURATION, not status:

- blue dashed ghost (accent-tinted, color-mix with an rgba fallback for old
  WebViews), clearly apart from the grey 'unavailable' icon;
- no state classes, no RGB tint, no alarm pulse, no active ripple on hidden
  devices — the only thing a ghost says is 'I am hidden, click to unhide'.

smoke_hidden_flag grew two assertions: the ghost carries no state classes
and is blue/dashed.

docs/TESTING.md

@@ -242,7 +242,9 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
 - [ ] Hide-from-plan flag (dev, docs/FILTERING.md): every device dialog has
       the checkbox, incl. virtual; hidden devices vanish from every mode and
       the count, still count toward room LQI, cast no glow/light fill; the
-      device editor's "Show hidden" (local, per tab) shows them ghosted;
+      device editor's "Show hidden" (local, per tab) shows them as BLUE
+      dashed ghosts — distinct from a grey unavailable icon — with NO live
+      state paint (no yellow, no alarm, no ripple);
       unticking keeps a hidden:false marker (re-seed protection); an old
       config materialises on first load by an editing client and legacy
       clients keep the old behaviour until then