Replace vanilla HTML with React + shadcn/ui + TailwindCSS scaffold

- Frontend: React 19 + Vite + TypeScript + TailwindCSS v4 + shadcn/ui
- Pre-installed shadcn/ui components: Button, Card, Input, Separator
- Multi-stage Dockerfile: build React client → serve with Express
- Server.js updated to serve built React SPA with API routes
- Sample landing page with shadcn/ui components demonstrating usage
- Path aliases (@/) configured in tsconfig + vite config
- components.json for easy `shadcn add <component>` usage

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

.dockerignore

@@ -1,6 +1,10 @@
 node_modules
+client/node_modules
+client/dist
 npm-debug.log
 .git
 .gitignore
 README.md
+SAAC_DEPLOYMENT.md
 .env
+.saac