Add core UI primitives in web/src/ui

Button, IconButton, Tag, Chip, TextInput, Segmented, Toggle, Checkbox,
Slider, Card and ListRow, styled with CSS Modules over the design tokens.
Every interactive primitive keeps a hit area of at least 44 px; visually
smaller controls draw their face inside a larger hit box.

Vitest now injects CSS Modules into jsdom (css: true) so tests assert the
sizes the stylesheets declare, alongside roles, aria states and keyboard
behaviour (Segmented arrow keys, Toggle Space/Enter, Chip "Remove X").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/ui/Checkbox.tsx

@@ -0,0 +1,17 @@
+import type { ComponentPropsWithRef, ReactNode } from 'react'
+import styles from './Checkbox.module.css'
+
+export type CheckboxProps = Omit<ComponentPropsWithRef<'input'>, 'type'> & {
+  label: ReactNode
+}
+
+// Native checkbox, styled. The wrapping label is the hit area (at least
+// 44×44), so tapping the text toggles it too.
+export function Checkbox({ label, className, ...rest }: CheckboxProps) {
+  return (
+    <label className={[styles.root, className].filter(Boolean).join(' ')}>
+      <input type="checkbox" className={styles.input} {...rest} />
+      <span className={styles.label}>{label}</span>
+    </label>
+  )
+}