feat(frontend): device pairing UI + live telemetry dashboard (Workstream H)

Adds /devices — pair an ESP32 sensor node (POST /api/device), reveal its
raw pairing token exactly once with a hard-to-miss "cannot be shown again"
warning (styled like a real API-key-reveal UI), then a live dashboard
subscribing to /ws/device-feed: the initial `devices` frame seeds paired
devices, and `reading` frames update one row per distinct sensor_type in
place. sensor_type/value/unit are rendered fully generically per the
contract (free-form, open-ended) — an unrecognized sensor_type renders
safely with no special-casing.

- frontend/src/lib/deviceFeed.ts: reconnecting WS client for
  /ws/device-feed, mirroring VeilSocket's backoff shape (receive-only, no
  outbox needed).
- frontend/src/pages/DevicesPage.{tsx,css}: pairing form + one-time token
  reveal + live device-card grid. Leans into "hacker" terminal styling
  (monospace readouts, terminal device cards) over the app's usual gothic
  chrome, per the design spec, while keeping the existing dark/violet
  palette tokens from App.css.
- Route + nav link wired into App.tsx / SeancePage.tsx.
- i18n: new `devices.*` / `nav.devices` keys in en.json + es.json; added a
  coverage-check.mjs domain rule for the dynamic connection-state key,
  mirroring the existing `seance.connection.` rule.

Tests: deviceFeed.test.ts (backoff/reconnect/frame delivery) and
DevicesPage.test.tsx (empty state, name validation, one-time token reveal
and dismissal, live frame updates in place without duplicating rows,
multi-device/multi-sensor rendering, and a mocked unrecognized sensor_type
that must not crash). Full suite: 154 passed (137 pre-existing + 17 new).
`npx tsc -b` and `npm run build` both clean.

Assumption (undocumented in spec): POST /api/device's JSON response shape
is inferred as `{id, name, token, last_seen_at}` since the Contract section
only describes the endpoint in prose. GET /api/device is intentionally not
called — the live dashboard is fully seeded by /ws/device-feed's initial
`devices` frame per the contract, so it's redundant for this page's scope.

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

frontend/src/i18n/en.json

@@ -13,6 +13,7 @@
     "home": "the threshold",
     "codex": "codex",
     "shop": "shop",
+    "devices": "devices",
     "seance": "séance",
     "enter": "enter",
     "logout": "depart"
@@ -333,6 +334,39 @@
     },
     "honesty": "the Ultimate Quantum Box is in active development — this is a reservation manifest, no payment is taken. the website works fully without it; hardware only knocks louder."
   },
+  "devices": {
+    "back": "← the threshold",
+    "title": "DEVICE BAY",
+    "subtitle": "pair hardware to your account and watch its telemetry live",
+    "pair": {
+      "title": "PAIR A NEW DEVICE",
+      "nameLabel": "device name",
+      "namePlaceholder": "e.g. attic-node-01",
+      "submit": "pair device",
+      "pending": "pairing…",
+      "nameRequired": "name the device before pairing",
+      "error": "the pairing handshake failed — try again"
+    },
+    "token": {
+      "warning": "copy this now — it cannot be shown again. losing it means re-pairing the device.",
+      "copy": "copy",
+      "copied": "copied",
+      "confirm": "I've saved it — hide token"
+    },
+    "dashboard": {
+      "title": "LIVE FEED",
+      "empty": "no devices paired yet — pair one above to see it here",
+      "lastSeen": "last seen",
+      "lastSeenNever": "never",
+      "noReadings": "awaiting first reading…",
+      "connection": {
+        "connecting": "reaching the feed…",
+        "open": "feed live",
+        "unstable": "feed unstable…",
+        "closed": "feed offline"
+      }
+    }
+  },
   "haunting": {
     "whisper": {
       "behindYou": "behind you",