swarm-ui: let a closed stream reconnect and a failed caps read retry

When the server answers an event stream with an error status the browser
gives up and the badge stayed on disconnected with nothing to press. The
terminal, its state header and the messages page now offer a reconnect,
and the one rule for the state after an error moved into a tested helper.
The resource caps panel gets a retry beside its error, and its loading
state announces itself.

frontend/packages/swarm-ui/src/agent/terminal/useSwarmAgentStateStream.ts

@@ -17,7 +17,12 @@
 // `status_text`/`running`).
 import { useEffect, useState } from "preact/hooks";
 
-export type ConnectionState = "connecting" | "open" | "reconnecting" | "closed";
+import {
+  connectionAfterError,
+  type ConnectionState,
+} from "../../ui/stream/connection.js";
+
+export type { ConnectionState };
 
 export interface TokenUsage {
   input_tokens?: number;
@@ -53,6 +58,8 @@ export interface AgentStateHeader {
 export interface UseSwarmAgentStateStreamResult {
   header: AgentStateHeader | null;
   connection: ConnectionState;
+  /** Opens the stream again, for one the browser gave up on. */
+  reconnect: () => void;
 }
 
 export function useSwarmAgentStateStream(
@@ -60,6 +67,8 @@ export function useSwarmAgentStateStream(
 ): UseSwarmAgentStateStreamResult {
   const [header, setHeader] = useState<AgentStateHeader | null>(null);
   const [connection, setConnection] = useState<ConnectionState>("connecting");
+  // Bumped by `reconnect`, which reruns the effect below.
+  const [attempt, setAttempt] = useState(0);
 
   useEffect(() => {
     setHeader(null);
@@ -78,13 +87,9 @@ export function useSwarmAgentStateStream(
         // badge cluster over — the next valid header replaces it.
       }
     };
-    es.onerror = () => {
-      setConnection(
-        es.readyState === 0 /* CONNECTING */ ? "reconnecting" : "closed",
-      );
-    };
+    es.onerror = () => setConnection(connectionAfterError(es.readyState));
     return () => es.close();
-  }, [streamUrl]);
+  }, [streamUrl, attempt]);
 
-  return { header, connection };
+  return { header, connection, reconnect: () => setAttempt((n) => n + 1) };
 }