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.css

@@ -0,0 +1,32 @@
+/* <FormField> — label stacked above its control, plus the shared
+   `.ui-form-control` chrome every text/select input in the kit draws
+   from (one class, so the two never drift). `max-width` + `width: 100%`
+   rather than a fixed `width`: caps the control on a wide desktop
+   viewport without forcing an overflow on a narrow/touch one.
+   `min-height` is a touch-target floor (44px at the default 16px root
+   font — WCAG 2.5.5's minimum), not a visual choice — it's the same on
+   every control in the kit whether or not it's ever used on a touch
+   device, since the alternative is a component that behaves differently
+   per input method. Colours are the shared base16-derived vars
+   (../../theme.css), never literal. */
+.ui-form-field {
+  display: flex;
+  flex-direction: column;
+  gap: 0.3em;
+}
+.ui-form-field-label {
+  font-size: 0.85em;
+  color: var(--muted);
+}
+.ui-form-control {
+  background: var(--bg);
+  color: var(--fg);
+  border: 1px solid var(--purple-dim);
+  border-radius: 0.3em;
+  padding: 0.4em 0.6em;
+  font: inherit;
+  width: 100%;
+  max-width: 16em;
+  box-sizing: border-box;
+  min-height: 2.75em;
+}

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>
+  );
+}