swarm UI: key LinkedAccounts by hive and agent

Selecting another agent kept the previous agent's rows on screen until
the new fetch landed. Keying both mounts by hive/agent remounts the
component on an agent change; a version bump still refetches in place.

Refs #4855

frontend/packages/swarm-ui/src/pages/agents/LinkedAccounts.tsx

@@ -5,6 +5,9 @@
 // Fetched on mount and again whenever `version` changes; `AgentsPage` bumps
 // it when a link dialog closes, so an account linked there shows up without
 // waiting for a reload.
+//
+// Its state belongs to one agent: callers key it by hive and agent, so
+// switching agents remounts it instead of showing the previous agent's rows.
 import { useEffect, useState } from "preact/hooks";
 import { readApiError, type ProblemDetails } from "@hive/shared/api-error.js";
 import { Badge } from "@hive/shared/badge.js";