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/navigation/RootNavigator.tsx

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+import React from 'react';
+import { createNativeStackNavigator } from '@react-navigation/native-stack';
+import { useDTTheme } from '@dangerousthings/react-native';
+import type { RootStackParamList } from './types';
+
+import { HomeScreen } from '../screens/HomeScreen';
+import { ButtonsScreen } from '../screens/ButtonsScreen';
+import { CardsScreen } from '../screens/CardsScreen';
+import { FormsScreen } from '../screens/FormsScreen';
+import { FeedbackScreen } from '../screens/FeedbackScreen';
+import { OverlaysScreen } from '../screens/OverlaysScreen';
+import { ThemeScreen } from '../screens/ThemeScreen';
+
+const Stack = createNativeStackNavigator<RootStackParamList>();
+
+export function RootNavigator() {
+  const theme = useDTTheme();
+
+  return (
+    <Stack.Navigator
+      initialRouteName="Home"
+      screenOptions={{
+        headerStyle: { backgroundColor: theme.colors.background },
+        headerTintColor: theme.custom.modeNormal,
+        headerTitleStyle: { fontWeight: '600' },
+        contentStyle: { backgroundColor: theme.colors.background },
+        animation: 'slide_from_right',
+      }}
+    >
+      <Stack.Screen
+        name="Home"
+        component={HomeScreen}
+        options={{ headerShown: false }}
+      />
+      <Stack.Screen
+        name="Buttons"
+        component={ButtonsScreen}
+        options={{ title: 'BUTTONS & ACTIONS' }}
+      />
+      <Stack.Screen
+        name="Cards"
+        component={CardsScreen}
+        options={{ title: 'CARDS & LABELS' }}
+      />
+      <Stack.Screen
+        name="Forms"
+        component={FormsScreen}
+        options={{ title: 'FORM CONTROLS' }}
+      />
+      <Stack.Screen
+        name="Feedback"
+        component={FeedbackScreen}
+        options={{ title: 'PROGRESS & FEEDBACK' }}
+      />
+      <Stack.Screen
+        name="Overlays"
+        component={OverlaysScreen}
+        options={{ title: 'OVERLAYS & NAV' }}
+      />
+      <Stack.Screen
+        name="Theme"
+        component={ThemeScreen}
+        options={{ title: 'THEME REFERENCE' }}
+      />
+    </Stack.Navigator>
+  );
+}

packages/showcase/mobile/src/navigation/navigationTheme.ts

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+import { DefaultTheme } from '@react-navigation/native';
+import { useBrand } from '../brand/BrandContext';
+
+export function useNavigationTheme() {
+  const { brand } = useBrand();
+  const colors = brand.dark;
+
+  return {
+    ...DefaultTheme,
+    dark: true,
+    colors: {
+      ...DefaultTheme.colors,
+      primary: colors.primary,
+      background: colors.bg,
+      card: colors.bg,
+      text: colors.textPrimary,
+      border: colors.primary,
+      notification: colors.secondary,
+    },
+  };
+}

packages/showcase/mobile/src/navigation/types.ts

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+export type RootStackParamList = {
+  Home: undefined;
+  Buttons: undefined;
+  Cards: undefined;
+  Forms: undefined;
+  Feedback: undefined;
+  Overlays: undefined;
+  Theme: undefined;
+};