feat: setup React + Vite + TypeScript frontend with Tailwind CSS

Initialize frontend infrastructure for Sprint 3 Landing Page:

Infrastructure Setup:
- Create React + Vite + TypeScript project in frontend/ directory
- Install Tailwind CSS, PostCSS, Autoprefixer dependencies
- Generate tailwind.config.js with content paths
- Generate postcss.config.js with plugins configuration
- Add Tailwind directives (@tailwind base/components/utilities) to src/index.css
- Update root .gitignore to exclude frontend/node_modules and build artifacts

Documentation Updates:
- Add CHANGELOG.md entry for Sprint 3 Landing Page development start
- Update README.md with frontend development instructions
- Add Node.js and npm prerequisites
- Document npm commands: install, dev, build, preview

Project Structure:
├── frontend/
│   ├── src/
│   │   ├── App.tsx
│   │   ├── main.tsx
│   │   ├── index.css (with Tailwind)
│   │   └── assets/
│   ├── index.html
│   ├── package.json
│   ├── tailwind.config.js
│   ├── postcss.config.js
│   └── vite.config.ts

Safety First:
- Verified directory doesn't exist before creation
- Checked all configuration files are valid
- Excluded node_modules from git
- No project code modified, only new frontend directory added

Refs: docs/frontend_landing_plan.md
Next: Start developing landing page components

frontend/src/App.tsx

@@ -0,0 +1,121 @@
+import { useState } from 'react'
+import reactLogo from './assets/react.svg'
+import viteLogo from './assets/vite.svg'
+import heroImg from './assets/hero.png'
+import './App.css'
+
+function App() {
+  const [count, setCount] = useState(0)
+
+  return (
+    <>
+      <section id="center">
+        <div className="hero">
+          <img src={heroImg} className="base" width="170" height="179" alt="" />
+          <img src={reactLogo} className="framework" alt="React logo" />
+          <img src={viteLogo} className="vite" alt="Vite logo" />
+        </div>
+        <div>
+          <h1>Get started</h1>
+          <p>
+            Edit <code>src/App.tsx</code> and save to test <code>HMR</code>
+          </p>
+        </div>
+        <button
+          className="counter"
+          onClick={() => setCount((count) => count + 1)}
+        >
+          Count is {count}
+        </button>
+      </section>
+
+      <div className="ticks"></div>
+
+      <section id="next-steps">
+        <div id="docs">
+          <svg className="icon" role="presentation" aria-hidden="true">
+            <use href="/icons.svg#documentation-icon"></use>
+          </svg>
+          <h2>Documentation</h2>
+          <p>Your questions, answered</p>
+          <ul>
+            <li>
+              <a href="https://vite.dev/" target="_blank">
+                <img className="logo" src={viteLogo} alt="" />
+                Explore Vite
+              </a>
+            </li>
+            <li>
+              <a href="https://react.dev/" target="_blank">
+                <img className="button-icon" src={reactLogo} alt="" />
+                Learn more
+              </a>
+            </li>
+          </ul>
+        </div>
+        <div id="social">
+          <svg className="icon" role="presentation" aria-hidden="true">
+            <use href="/icons.svg#social-icon"></use>
+          </svg>
+          <h2>Connect with us</h2>
+          <p>Join the Vite community</p>
+          <ul>
+            <li>
+              <a href="https://github.com/vitejs/vite" target="_blank">
+                <svg
+                  className="button-icon"
+                  role="presentation"
+                  aria-hidden="true"
+                >
+                  <use href="/icons.svg#github-icon"></use>
+                </svg>
+                GitHub
+              </a>
+            </li>
+            <li>
+              <a href="https://chat.vite.dev/" target="_blank">
+                <svg
+                  className="button-icon"
+                  role="presentation"
+                  aria-hidden="true"
+                >
+                  <use href="/icons.svg#discord-icon"></use>
+                </svg>
+                Discord
+              </a>
+            </li>
+            <li>
+              <a href="https://x.com/vite_js" target="_blank">
+                <svg
+                  className="button-icon"
+                  role="presentation"
+                  aria-hidden="true"
+                >
+                  <use href="/icons.svg#x-icon"></use>
+                </svg>
+                X.com
+              </a>
+            </li>
+            <li>
+              <a href="https://bsky.app/profile/vite.dev" target="_blank">
+                <svg
+                  className="button-icon"
+                  role="presentation"
+                  aria-hidden="true"
+                >
+                  <use href="/icons.svg#bluesky-icon"></use>
+                </svg>
+                Bluesky
+              </a>
+            </li>
+          </ul>
+        </div>
+      </section>
+
+      <div className="ticks"></div>
+      <section id="spacer"></section>
+    </>
+  )
+}
+
+export default App