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/file-pane.tsx

@@ -119,7 +119,7 @@ function FilePreviewBody({
   filePath,
 }: FilePreviewBodyProps) {
   const { theme } = useUnistyles();
-  const isDark = theme.colors.surface0 === "#181B1A";
+  const isDark = theme.colorScheme === "dark";
   const colorMap = isDark ? darkHighlightColors : lightHighlightColors;
   const baseColor = isDark ? "#c9d1d9" : "#24292f";