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/vite.config.ts

@@ -1,5 +1,4 @@
-/// <reference types="vitest" />
-import { defineConfig } from 'vite'
+import { defineConfig } from 'vitest/config'
 import react from '@vitejs/plugin-react'
 
 // https://vite.dev/config/