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/DevPanel.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react';
-import { api, Match } from '../api/client';
+import { Match } from '../api/client';
 import styles from './DevPanel.module.css';
 
 const DEV_USERS = [