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/components/LoginPage.tsx

@@ -1,4 +1,5 @@
-import { useState, FormEvent } from 'react'
+import { useState } from 'react'
+import type { FormEvent } from 'react'
 
 interface LoginPageProps {
   onLogin: (email: string, password: string) => Promise<void>