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/AgentsPage.tsx

@@ -394,6 +394,7 @@ export function AgentsPage() {
         <>
           {a.hive ? (
             <LinkedAccounts
+              key={`${a.hive}/${a.name}`}
               hive={a.hive}
               agent={a.name}
               version={linkVersion}
@@ -600,6 +601,7 @@ export function AgentsPage() {
                   <dd>
                     {detailTarget.hive ? (
                       <LinkedAccounts
+                        key={`${detailTarget.hive}/${detailTarget.name}`}
                         hive={detailTarget.hive}
                         agent={detailTarget.name}
                         version={linkVersion}

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";