Add desktop (Electron) and mobile (Expo) showcase apps

New workspace packages for demonstrating design system components:
- Desktop: Electron + Vite app with brand/theme switching
- Mobile: Expo React Native app with brand switching

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/showcase/mobile/src/brand/BrandContext.tsx

@@ -0,0 +1,42 @@
+import React, { createContext, useContext, useState, useMemo, ReactNode } from 'react';
+import { brands, themes } from '@dangerousthings/tokens';
+import type { ThemeBrand, BrandTokens } from '@dangerousthings/tokens';
+import { buildThemeFromBrand } from './buildTheme';
+import type { DTExtendedTheme } from '@dangerousthings/react-native';
+
+interface BrandContextValue {
+  brandId: ThemeBrand;
+  brand: BrandTokens;
+  theme: DTExtendedTheme;
+  setBrand: (id: ThemeBrand) => void;
+  availableBrands: typeof themes;
+}
+
+const BrandContext = createContext<BrandContextValue>(null!);
+
+export function useBrand() {
+  return useContext(BrandContext);
+}
+
+interface BrandProviderProps {
+  children: ReactNode;
+  initialBrand?: ThemeBrand;
+}
+
+export function BrandProvider({ children, initialBrand = 'dt' }: BrandProviderProps) {
+  const [brandId, setBrandId] = useState<ThemeBrand>(initialBrand);
+
+  const value = useMemo(() => {
+    const brand = brands[brandId] as BrandTokens;
+    const theme = buildThemeFromBrand(brand);
+    return {
+      brandId,
+      brand,
+      theme,
+      setBrand: setBrandId,
+      availableBrands: themes,
+    };
+  }, [brandId]);
+
+  return <BrandContext.Provider value={value}>{children}</BrandContext.Provider>;
+}