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/ui/button.tsx

@@ -136,9 +136,11 @@ export function Button({
       return <View>{leftIcon}</View>;
     }
 
-    const color = variant === "ghost"
-      ? (isGhostHovered ? theme.colors.foreground : theme.colors.foregroundMuted)
-      : theme.colors.foreground;
+    const color = variant === "default"
+      ? theme.colors.accentForeground
+      : variant === "ghost"
+        ? (isGhostHovered ? theme.colors.foreground : theme.colors.foregroundMuted)
+        : theme.colors.foreground;
     const iconSize = ICON_SIZE[size];
 
     // Render function