fix(app): improve light mode theming

- Make PaseoLogo theme-aware (uses foreground color instead of hardcoded white)
- Add shadow tokens (sm/md/lg) to theme with per-scheme opacity, radius, and offset
- Replace all 16 hardcoded shadow instances with spreadable theme.shadow tokens
- Fix button icon color for default variant (use accentForeground, not foreground)
- Fix dark background flash on startup (root layout used hardcoded darkTheme)
- Add theme.colorScheme to replace fragile hex-string dark mode detection
- Add scrollbarHandle and surfaceWorkspace tokens to eliminate isDark branching

packages/app/src/components/welcome-screen.tsx

@@ -320,7 +320,7 @@ export function WelcomeScreen({ onHostAdded }: WelcomeScreenProps) {
       testID="welcome-screen"
     >
       <View style={styles.content}>
-        <PaseoLogo size={96} color={theme.colors.foreground} />
+        <PaseoLogo size={96} />
         <Text style={styles.title}>Welcome to Paseo</Text>
         <Text style={styles.subtitle}>
           {showHostList ? "Connecting to your hosts…" : "Connect to your host to start"}