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/desktop/updates/update-banner.tsx

@@ -128,11 +128,7 @@ const styles = StyleSheet.create((theme) => ({
     paddingVertical: theme.spacing[3],
     paddingLeft: theme.spacing[4],
     paddingRight: theme.spacing[3],
-    shadowColor: "#000",
-    shadowOffset: { width: 0, height: 4 },
-    shadowOpacity: 0.2,
-    shadowRadius: 12,
-    elevation: 8,
+    ...theme.shadow.md,
     maxWidth: 480,
   },
   closeButton: {