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/i18n/en.json

@@ -258,6 +258,14 @@
         "body": "a diagnostic pulse only — nothing risked, nothing gained."
       }
     },
+    "whisper": {
+      "title": "THE VESSEL WHISPERS",
+      "online": "the vessel is awake",
+      "offline": "the vessel sleeps",
+      "waiting": "no whisper yet…",
+      "coldSpot": "❄ a cold spot gathers around the vessel",
+      "pressureAnomaly": "◎ the air presses close — the atmosphere shifts"
+    },
     "toast": {
       "rate_limited": "the veil is thin — slow your knocking",
       "veil_crowded": "too many spirits crowd the line"

frontend/src/i18n/es.json

@@ -258,6 +258,14 @@
         "body": "solo un pulso de diagnóstico — no se arriesga ni se gana nada."
       }
     },
+    "whisper": {
+      "title": "EL RECIPIENTE SUSURRA",
+      "online": "el recipiente está despierto",
+      "offline": "el recipiente duerme",
+      "waiting": "aún ningún susurro…",
+      "coldSpot": "❄ un punto frío se acumula en torno al recipiente",
+      "pressureAnomaly": "◎ el aire se cierra — la atmósfera cambia"
+    },
     "toast": {
       "rate_limited": "el velo es delgado — golpea más despacio",
       "veil_crowded": "demasiados espíritus se apiñan en la línea"