feat: a GPU agent on the CPU shows as degraded, in the System view and on its own page (4410)

torch and onnxruntime both fall back to the CPU without raising, so the agent
that ran CPU-bound for weeks after a driver update leased and checked in like
a healthy one.

- The agent sends its startup accel report on every lease and heartbeat.
- The server keeps a bounded copy on the roster row. A running agent with a
  runtime off the GPU becomes `degraded`, with a sentence naming the runtime
  and the reason.
- The top nav shows it amber.
- The agent page carries a banner, and its pill reads "CPU only".

Also: the bandwidth field gets the page's − / + stepper, and both number
fields drop the browser's spin arrows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/src/utils/systemParts.js

@@ -24,7 +24,8 @@ export function queueKey(queues) {
 }
 
 // Worst first. A stopped datastore is why someone opened this tab.
-export const SEVERITY = { down: 3, stale: 2, unknown: 1, ok: 0 }
+// `degraded`: checking in, but slow — a GPU agent on the CPU (#4410).
+export const SEVERITY = { down: 4, stale: 3, degraded: 2, unknown: 1, ok: 0 }
 
 export function kindLabel(kind) {
   if (kind === 'celery') return 'worker lane'