fix(agents): use icon-only composer actions (#613)

The composers' primary actions were text buttons. Start, Send and Queue, plus Send in edit-and-resend, now share an icon-only submit button that keeps the translated name as its accessible label and tooltip, and shows a spinner while the action is pending. Stop is unchanged.

src/features/agents/SubmitButton.module.css

@@ -0,0 +1,4 @@
+.submit {
+  width: var(--h-control);
+  padding: 0;
+}