feat: the vessel speaks in the séance — DeviceWhisper panel

Workstream D of the usability wave (#8). A compact live panel in the
séance side column, shown only when the seeker has paired hardware:
device name, online dot (reading within 60s), latest readings in the
terminal readout style, and cold-spot / pressure-anomaly flags from the
lib/coldSpot.ts cores. Fetches GET /api/device once on mount; renders
nothing on zero devices or fetch failure — the séance never sees an
error from this panel. Socket closed on unmount.

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

frontend/src/i18n/en.json

@@ -257,6 +257,14 @@
         "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"