style: CSS polish, light mode verification, build fixes

- Add --primary-rgb, --transition-fast, --transition-normal CSS tokens to :root
- Add --primary-rgb override in [data-theme="light"] section
- Fix TS error: remove unused devUser prop from Route elements in App.tsx (API patching via window._devUser makes props redundant)
- Fix TS error: remove unused 'api' import from DevPanel.tsx

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/Toast.js

@@ -0,0 +1,10 @@
+import { jsx as _jsx } from "react/jsx-runtime";
+import { useEffect } from 'react';
+import styles from './Toast.module.css';
+export default function Toast({ message, onDismiss, duration = 5000 }) {
+    useEffect(() => {
+        const timer = setTimeout(onDismiss, duration);
+        return () => clearTimeout(timer);
+    }, [onDismiss, duration]);
+    return (_jsx("div", { className: styles.toast, onClick: onDismiss, children: message }));
+}