feat(gpu): GPU agent admin card — token, queue, backfill (#114)

The FC-side control surface the operator asked for: Settings → Tagging → "GPU
agent". Generate/reveal/copy/rotate the agent bearer token (with the FC URL to
point the agent at), see the live job-queue depth (pending/in-flight/done/
errored, polled), and a "Queue character embedding (CCIP)" button that triggers
the library backfill. Plain-HTTP-safe copy (copyText resolves on success,
throws on fail). Closes the "how do I get the token in the UI" gap.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

frontend/src/components/settings/MaintenancePanel.vue

@@ -27,6 +27,7 @@
       <div class="fc-tile-stack">
         <MLThresholdSliders />
         <HeadsCard />
+        <GpuAgentCard />
         <AllowlistTable />
         <AliasTable />
         <TagEvalCard />
@@ -54,6 +55,7 @@ import MissingFileRepairCard from './MissingFileRepairCard.vue'
 import DbMaintenanceCard from './DbMaintenanceCard.vue'
 import MLThresholdSliders from './MLThresholdSliders.vue'
 import HeadsCard from './HeadsCard.vue'
+import GpuAgentCard from './GpuAgentCard.vue'
 import AllowlistTable from './AllowlistTable.vue'
 import AliasTable from './AliasTable.vue'
 import TagEvalCard from './TagEvalCard.vue'