Deploy web app to satsprice.app/app/ and link it from the website

The GitHub Pages workflow now builds the Compose wasmJs web app and
publishes it alongside the marketing site, so it's reachable at
/app/. Add nav/hero/download links to it and list Web among the
supported platforms. Add website/serve-local.sh to reproduce the
production layout locally, and document the setup in README/AGENTS.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

website/index.html

@@ -4,7 +4,7 @@
 <meta charset="UTF-8" />
 <meta name="viewport" content="width=device-width, initial-scale=1.0" />
 <title>SatsPrice — Bitcoin Price &amp; Sats Converter</title>
-<meta name="description" content="SatsPrice converts Bitcoin, satoshis, and fiat currency in real time, sourced from Coinbase and CoinGecko. Free and open source for Android, iOS, and macOS." />
+<meta name="description" content="SatsPrice converts Bitcoin, satoshis, and fiat currency in real time, sourced from Coinbase and CoinGecko. Free and open source for Android, iOS, macOS, and Web." />
 <link rel="icon" href="assets/app-icon.png" type="image/png" />
 <link rel="stylesheet" href="styles.css" />
 </head>
@@ -22,6 +22,7 @@
       <a href="#features">Features</a>
       <a href="#platforms">Screenshots</a>
       <a href="#download">Download</a>
+      <a href="/app/" target="_blank" rel="noopener">Web App</a>
       <a href="https://github.com/tyiu/sats-price" target="_blank" rel="noopener">Code</a>
     </nav>
 
@@ -37,6 +38,7 @@
     <a href="#features">Features</a>
     <a href="#platforms">Screenshots</a>
     <a href="#download">Download</a>
+    <a href="/app/" target="_blank" rel="noopener">Web App</a>
     <a href="https://github.com/tyiu/sats-price" target="_blank" rel="noopener">GitHub</a>
   </div>
 </header>
@@ -60,6 +62,11 @@
           <a href="#platforms" class="btn btn-ghost">See screenshots</a>
         </div>
 
+        <p class="hero-webapp">
+          Don't want to install anything?
+          <a href="/app/" target="_blank" rel="noopener">Try it in your browser →</a>
+        </p>
+
         <div class="platform-pills">
           <span class="pill">
             <svg viewBox="0 0 24 24" fill="currentColor"><path d="M17.05 12.536c-.03-3.06 2.5-4.53 2.61-4.6-1.42-2.08-3.63-2.37-4.42-2.4-1.88-.19-3.68 1.11-4.63 1.11-.96 0-2.42-1.09-3.98-1.06-2.05.03-3.94 1.19-5 3.02-2.13 3.7-.55 9.18 1.53 12.18 1.02 1.47 2.23 3.11 3.82 3.05 1.53-.06 2.11-.99 3.96-.99 1.84 0 2.37.99 3.98.96 1.65-.03 2.69-1.5 3.7-2.98 1.17-1.71 1.65-3.37 1.67-3.45-.04-.02-3.2-1.23-3.24-4.84zM14.15 3.6c.84-1.02 1.41-2.43 1.25-3.6-1.22.06-2.69.84-3.55 1.85-.78.9-1.46 2.34-1.28 3.57 1.2.09 2.75-.61 3.58-1.82z"/></svg>
@@ -73,6 +80,10 @@
             <svg viewBox="0 0 24 24" fill="currentColor"><path d="M17.6 9.48l1.84-3.18c.16-.31.04-.69-.26-.85-.29-.15-.65-.06-.83.22l-1.88 3.24a11.43 11.43 0 0 0-8.94 0L5.65 5.67c-.18-.28-.54-.37-.83-.22-.3.16-.42.54-.26.85L6.4 9.48C3.3 11.25 1.28 14.44 1 18h22c-.28-3.56-2.3-6.75-5.4-8.52zM7 15.25a1.25 1.25 0 1 1 0-2.5 1.25 1.25 0 0 1 0 2.5zm10 0a1.25 1.25 0 1 1 0-2.5 1.25 1.25 0 0 1 0 2.5z"/></svg>
             Android
           </span>
+          <span class="pill">
+            <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><circle cx="12" cy="12" r="10"/><line x1="2" y1="12" x2="22" y2="12"/><path d="M12 2a15.3 15.3 0 0 1 4 10 15.3 15.3 0 0 1-4 10 15.3 15.3 0 0 1-4-10 15.3 15.3 0 0 1 4-10z"/></svg>
+            Web
+          </span>
         </div>
 
         <div class="trust-row">
@@ -162,7 +173,7 @@
         <div class="feature-card wide">
           <div class="icon">💻</div>
           <h3>Native on every platform</h3>
-          <p>SwiftUI on iOS and macOS, Compose Multiplatform on Android — built with Kotlin Multiplatform to feel right at home on each device, not like a port.</p>
+          <p>SwiftUI on iOS and macOS, Compose Multiplatform on Android and Web — built with Kotlin Multiplatform to feel right at home on each device, not like a port.</p>
         </div>
 
         <div class="feature-card">
@@ -279,10 +290,17 @@
           </a>
         </div>
 
+        <div class="webapp-divider"><span>or</span></div>
+
+        <div class="webapp-cta">
+          <a href="/app/" class="btn btn-ghost" target="_blank" rel="noopener">Open the web app — no install required</a>
+        </div>
+
         <div class="version-row">
           <span class="pill">🍎 iOS 16.0+</span>
           <span class="pill">🖥️ macOS 13.0+</span>
           <span class="pill">🤖 Android 7.0+</span>
+          <span class="pill">🌐 Web — any modern browser</span>
         </div>
 
         <p class="oss-note">