Sensors: read-only tokens, live stream, Prometheus, status sensors, widgets, embeds

- sensors:read tokens open the sensor endpoints (here and on managed machines
  through the hub) and nothing else; full tokens are never accepted from a URL
- /api/sensors/stream: a shaped query that keeps answering, over SSE
- /metrics: Prometheus text format
- services, updates, ZFS and SMART as background status sensors
- widget library over the sensor API; custom widgets on the dashboard
- /embed/widget: one live widget in its own bundle, the only frameable path
- Dashboard tab is now Monitors

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

web/src/TerminalPane.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useRef, useState } from 'react'
 import { Terminal } from '@xterm/xterm'
+import '@xterm/xterm/css/xterm.css'
 import { FitAddon } from '@xterm/addon-fit'
 import { WebLinksAddon } from '@xterm/addon-web-links'
 import { wsURL } from './api'