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/web-desktop-scrollbar.tsx

@@ -350,8 +350,7 @@ export function WebDesktopScrollbarOverlay({
         ? HANDLE_OPACITY_VISIBLE
         : 0;
   const handleWidth = isDragging || isHandleHovered ? HANDLE_WIDTH_ACTIVE : HANDLE_WIDTH_IDLE;
-  const isDark = theme.colors.surface0 === "#181B1A";
-  const handleColor = isDark ? theme.colors.palette.zinc[500] : theme.colors.palette.zinc[700];
+  const handleColor = theme.colors.scrollbarHandle;
   const handleCursor = isDragging ? "grabbing" : "grab";
   const handleTravelDurationMs =
     isDragging || isScrollActive ? 0 : HANDLE_TRAVEL_TRANSITION_DURATION_MS;