feat: App shell with login, session sidebar, chat panel, right panel

- Create RightPanel component with browser/artifacts tab switching
- Replace App.tsx with 3-column layout (auth loading/login/app states)
- Replace main.tsx with globals.css import (remove index.css)
- Remove Vite template files (App.css, index.css, assets/react.svg)
- Fix LoginPage.tsx FormEvent type-only import for verbatimModuleSyntax
- Fix vite.config.ts to use vitest/config for proper test property typing
- Add tests: RightPanel (10 tests), App rendering states (7 tests)

frontend/src/main.tsx

@@ -1,7 +1,7 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
-import './index.css'
-import App from './App.tsx'
+import App from './App'
+import './styles/globals.css'
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>