swarm-ui: the terminal message box has a placeholder and a spinner instead of a label and a sent line

The label 'message <agent>' is the input's placeholder (the label stays for
screen readers). The send button shows a spinner over its label while the
request is pending and keeps its width, and the 'sent' line is gone: the box
empties when the message is held. Spinner is a component of its own now, used
by the button and the refresh picker.

frontend/packages/swarm-ui/src/agent/terminal/AgentInputForm.css

@@ -11,6 +11,3 @@
 .ui-agent-input-form > :first-child {
   flex: 1;
 }
-.ui-agent-input-sent {
-  margin-block-start: 0.25rem;
-}

frontend/packages/swarm-ui/src/agent/terminal/AgentInputForm.tsx

@@ -7,7 +7,6 @@ import { useState } from "preact/hooks";
 import { ApiErrorPanel } from "@hive/shared/api-error-panel.js";
 import { TextField } from "../../ui/text-field/TextField.js";
 import { Button } from "../../ui/button/Button.js";
-import { Hint } from "../../ui/hint/Hint.js";
 import { useSubmit } from "../../ui/submit/useSubmit.js";
 import "./AgentInputForm.css";
 
@@ -34,19 +33,16 @@ export function AgentInputForm({ agentName }: { agentName: string }) {
         <TextField
           id={`agent-input-${agentName}`}
           label={`message ${agentName}`}
+          hideLabel
+          placeholder={`message ${agentName}`}
           value={body}
           required
           onInput={setBody}
         />
-        <Button variant="primary" type="submit" disabled={submitting}>
-          {submitting ? "sending…" : "send"}
+        <Button variant="primary" type="submit" busy={submitting}>
+          send
         </Button>
       </form>
-      {state.status === "done" && (
-        <Hint tone="ok" class="ui-agent-input-sent">
-          sent
-        </Hint>
-      )}
       {state.status === "error" && (
         <ApiErrorPanel context="failed to send" problem={state.problem} />
       )}