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/agent-stream-view.tsx

@@ -1127,14 +1127,7 @@ const stylesheet = StyleSheet.create((theme) => ({
     backgroundColor: theme.colors.surface2,
     alignItems: "center",
     justifyContent: "center",
-    shadowColor: "#000",
-    shadowOffset: {
-      width: 0,
-      height: 2,
-    },
-    shadowOpacity: 0.25,
-    shadowRadius: 3.84,
-    elevation: 5,
+    ...theme.shadow.sm,
   },
   scrollToBottomIcon: {
     color: theme.colors.foreground,