feat: read-only device list endpoint + PWA section (#52)

[mdj2812-hephaestus]

What

Closes #52. Registered ESP32 buttons are now visible in the PWA as a read-only section, backed by a new read endpoint.

Endpoint — GET /api/home_intercom/devices (HA) and GET /devices + the HA-style alias (Docker) return the registry snapshot keyed by MAC:

{
  "AA:BB:CC:DD:EE:FF": {
    "name": "Kitchen Button",
    "room": "living",
    "last_seen": "2026-07-24T08:00:00",
    "firmware_version": "1.0.0",
    "revoked": false
  }
}

PWA — an "Intercom Buttons" section under the room grid: name, bound room, firmware · last-seen per row; revoked buttons struck through. Hidden when the registry is empty or the endpoint is unavailable.

Design notes

• Auth follows the existing split: the HA view is gated by the PWA shared token (same as RecordView) — unlike /rooms, this listing exposes device names and MACs, which are the registry's only auth material. The Docker route stays LAN-open, same trust model as /record.
• Payload built by shared devices_payload() in shared.py so both deployment targets stay identical.
• Read-only by design: rename / rebind / revoke live in the HA options flow (Device management UI: list / rename / rebind / revoke registered devices #48), which is where HA users expect management.
• Renders via textContent only (no innerHTML); re-renders on language switch through a renderDevices hook in i18n.js.

Verification

• 190 tests pass, coverage 93.72%; ruff check + ruff format clean
• Unit: TestDevicesView (token gate → 401, payload with valid token, {} without store) + Docker route tests (empty / registered / HA alias, isolated store)
• Smoke, both targets: Docker asserts the hello-registered MAC appears in /devices and the alias matches; HA extracts the PWA token from .storage and asserts 401-without-token + listing with token
• Local Docker smoke: green (incl. new checks)