feat: the vessel whispers in the séance (Workstream D)

DeviceWhisper: a compact live panel in the séance side column, shown only
when the seeker has paired hardware. One DeviceFeedSocket, readings folded
through the coldSpot cores, online dot with a 15s tick so a silent device
goes dark, cold-spot/pressure flags when active. Renders nothing on zero
devices or any fetch failure — the séance never errors because of this
panel. Socket closed on unmount.

Uses the real GET /api/device endpoint — the spec said /api/devices,
which does not exist; the agent verified against routes/device.py rather
than trusting the spec.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/SeancePage.tsx

@@ -24,6 +24,7 @@ import { GraphView } from '../components/GraphView'
 import { Transcript } from '../components/Transcript'
 import { EntityCard } from '../components/EntityCard'
 import { RitualPanel } from '../components/RitualPanel'
+import { DeviceWhisper } from '../components/DeviceWhisper'
 import { JudgmentPanel } from '../components/JudgmentPanel'
 import { TelemetryReadout } from '../components/TelemetryReadout'
 import { PlanchetteMachine } from '../lib/planchette'
@@ -382,6 +383,7 @@ function SeanceSession({ username }: { username: string }) {
             onToggleOpen={() => setDossierOpen((o) => !o)}
           />
 
+          <DeviceWhisper />
           <RitualPanel />
           <JudgmentPanel />