feat(desktop): port the 15 design-system primitives (#14)

Verbatim port of the handoff primitives into components/ui/ — Icon,
Button, IconButton, Badge, Tag, Card, Tabs (core); Input, Select,
Checkbox, Radio, Switch (forms); Dialog, Toast, Tooltip (feedback).
Each keeps its injected token-referencing CSS byte-for-byte; the
handoff .d.ts contracts become the exported prop interfaces. Barrel at
components/ui/index.ts.

Adds a PrimitivesGallery (app root for now; real shell is P3-2) that
exercises every primitive with a light/dark toggle. Smoke suite asserts
the gallery, section coverage, theme flip, and dialog open/Escape;
screenshots captured for both themes. typecheck + 5 e2e green.

Closes P3-1.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/desktop/src/renderer/src/components/ui/checkbox.tsx

@@ -0,0 +1,54 @@
+import React from 'react'
+import { Icon } from './icon.js'
+
+/**
+ * Checkbox with label; 16px box, spruce when checked.
+ */
+export interface CheckboxProps {
+  label?: React.ReactNode;
+  checked?: boolean;
+  onChange?: (e: React.ChangeEvent<HTMLInputElement>) => void;
+  disabled?: boolean;
+  style?: React.CSSProperties;
+}
+
+const CSS = `
+.ct-check { display: inline-flex; align-items: center; gap: 9px; cursor: pointer; font: var(--text-body); color: var(--ink-1); }
+.ct-check--disabled { opacity: 0.5; cursor: not-allowed; }
+.ct-check__input { position: absolute; opacity: 0; width: 0; height: 0; }
+.ct-check__box {
+  width: 16px; height: 16px; flex-shrink: 0;
+  display: inline-flex; align-items: center; justify-content: center;
+  background: var(--surface-card);
+  border: 1px solid var(--line-2);
+  border-radius: var(--radius-1);
+  color: transparent;
+  transition: background var(--duration-fast) var(--ease-out), border-color var(--duration-fast) var(--ease-out);
+}
+.ct-check:hover:not(.ct-check--disabled) .ct-check__box { border-color: var(--border-strong); }
+.ct-check__input:checked + .ct-check__box { background: var(--accent); border-color: var(--accent); color: var(--ink-inverse); }
+.ct-check__input:focus-visible + .ct-check__box { outline: 2px solid var(--focus-ring); outline-offset: 2px; }
+`;
+(function inject() {
+  if (typeof document !== 'undefined' && !document.getElementById('ct-check-css')) {
+    const s = document.createElement('style'); s.id = 'ct-check-css'; s.textContent = CSS;
+    document.head.appendChild(s);
+  }
+})();
+
+export function Checkbox({ label, checked, onChange, disabled = false, style, ...rest }: CheckboxProps) {
+  return (
+    <label className={`ct-check${disabled ? ' ct-check--disabled' : ''}`} style={style}>
+      <input
+        type="checkbox"
+        className="ct-check__input"
+        checked={checked}
+        onChange={onChange}
+        disabled={disabled}
+        {...rest}
+      />
+      <span className="ct-check__box"><Icon name="check" size={12} strokeWidth={2.5} /></span>
+      {label ? <span>{label}</span> : null}
+    </label>
+  );
+}