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/button/Button.css

@@ -0,0 +1,26 @@
+/* <Button> — same touch-target floor as `.ui-form-control`
+   (../form-field/FormField.css: 2.75em ≈ 44px at the default root font,
+   WCAG 2.5.5) so a button never reads as a smaller tap target than the
+   input sitting next to it. Colours are the shared base16-derived vars
+   (../../theme.css). */
+.ui-button {
+  border-radius: 0.3em;
+  padding: 0.5em 1.2em;
+  font: inherit;
+  cursor: pointer;
+  min-height: 2.75em;
+}
+.ui-button-primary {
+  background: var(--purple-dim);
+  color: var(--fg);
+  border: none;
+}
+.ui-button-default {
+  background: var(--bg-elev);
+  color: var(--fg);
+  border: 1px solid var(--border);
+}
+.ui-button:disabled {
+  opacity: 0.6;
+  cursor: default;
+}

frontend/packages/swarm-ui/src/ui/button/Button.tsx

@@ -0,0 +1,35 @@
+// <Button> — the shared clickable-action control (form submit today,
+// any future toolbar/dialog action later). `variant` is `'primary'`
+// (the one emphasised action on a page, e.g. a form's submit) or
+// `'default'` (everything else) — two is enough for every real caller
+// so far; a third reading gets added the day something needs it, not
+// speculatively.
+import type { ComponentChildren } from 'preact';
+import './Button.css';
+
+export type ButtonVariant = 'primary' | 'default';
+
+export function Button({
+  variant = 'default',
+  type = 'button',
+  disabled,
+  onClick,
+  children,
+}: {
+  variant?: ButtonVariant;
+  type?: 'button' | 'submit';
+  disabled?: boolean;
+  onClick?: (e: Event) => void;
+  children: ComponentChildren;
+}) {
+  return (
+    <button
+      class={'ui-button ui-button-' + variant}
+      type={type}
+      disabled={disabled}
+      onClick={onClick}
+    >
+      {children}
+    </button>
+  );
+}