swarm-ui: shared form-field kit (TextField, SelectField, Button)

Closes #3448.

New ui/ primitives: FormField (shared label+control wrapper),
TextField, SelectField, Button — each with a min-height touch target
(2.75em ~= 44px, WCAG 2.5.5) per mara's #3447 ask, and a max-width
instead of a fixed width so the control caps on desktop without
overflowing a narrow/touch viewport.

CreateAgentPage's name field + submit button now come from the kit
instead of page-scoped CSS; ComponentsPage gets a section for each new
primitive with an editable sample.

frontend/packages/swarm-ui/src/ui/form-field/FormField.tsx

@@ -0,0 +1,27 @@
+// <FormField> — label + control wrapper shared by every labelled form
+// control (`TextField`, `SelectField`, …). Not exported as a standalone
+// primitive callers reach for directly — it only exists so those two
+// don't each reinvent the label/spacing chrome; a bare labelled `<div>`
+// wrapper has no independent identity worth a `/components` entry of
+// its own.
+import type { ComponentChildren } from 'preact';
+import './FormField.css';
+
+export function FormField({
+  label,
+  htmlFor,
+  children,
+}: {
+  label: string;
+  htmlFor: string;
+  children: ComponentChildren;
+}) {
+  return (
+    <div class="ui-form-field">
+      <label class="ui-form-field-label" for={htmlFor}>
+        {label}
+      </label>
+      {children}
+    </div>
+  );
+}