docs: add Google Play Store link to nostrkey.com

Add Android download card pointing to Play Store listing,
update Cross-Platform feature text and meta descriptions
to include Android.

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

Author: vveerrgg

docs/index.html

@@ -4,13 +4,13 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>NostrKey - Cross-Browser Nostr Key Management</title>
-    <meta name="description" content="Secure Nostr key management for Chrome, Safari, and iOS. NIP-07 signing, encrypted vault, and nsecBunker support.">
+    <meta name="description" content="Secure Nostr key management for Chrome, Safari, iOS, and Android. NIP-07 signing, encrypted vault, and nsecBunker support.">
 
     <!-- Open Graph / Facebook -->
     <meta property="og:type" content="website">
     <meta property="og:url" content="https://nostrkey.com/">
     <meta property="og:title" content="NostrKey - Cross-Browser Nostr Key Management">
-    <meta property="og:description" content="Secure Nostr key management for Chrome, Safari, and iOS. NIP-07 signing, encrypted vault, and nsecBunker support.">
+    <meta property="og:description" content="Secure Nostr key management for Chrome, Safari, iOS, and Android. NIP-07 signing, encrypted vault, and nsecBunker support.">
     <meta property="og:image" content="https://nostrkey.com/og-image.png">
     <meta property="og:image:width" content="1200">
     <meta property="og:image:height" content="630">
@@ -19,7 +19,7 @@
     <meta name="twitter:card" content="summary_large_image">
     <meta name="twitter:url" content="https://nostrkey.com/">
     <meta name="twitter:title" content="NostrKey - Cross-Browser Nostr Key Management">
-    <meta name="twitter:description" content="Secure Nostr key management for Chrome, Safari, and iOS. NIP-07 signing, encrypted vault, and nsecBunker support.">
+    <meta name="twitter:description" content="Secure Nostr key management for Chrome, Safari, iOS, and Android. NIP-07 signing, encrypted vault, and nsecBunker support.">
     <meta name="twitter:image" content="https://nostrkey.com/og-image.png">
 
     <!-- Favicon -->
@@ -365,7 +365,7 @@ <h4>👤 Multi-Profile</h4>
                 </div>
                 <div class="feature">
                     <h4>📱 Cross-Platform</h4>
-                    <p>Chrome, Safari (macOS + iOS) support</p>
+                    <p>Chrome, Safari (macOS + iOS), and Android</p>
                 </div>
                 <div class="feature">
                     <h4>🔓 Open Source</h4>
@@ -393,6 +393,11 @@ <h4>Safari (macOS)</h4>
                     <h4>Safari (iOS)</h4>
                     <p>Xcode build</p>
                 </a>
+                <a href="https://play.google.com/store/apps/details?id=com.nostrkey.app" target="nostrkey-web" class="download-card">
+                    <svg viewBox="0 0 24 24" fill="#3DDC84"><path d="M3 20.5V3.5a1 1 0 011.5-.87l15 8.5a1 1 0 010 1.74l-15 8.5A1 1 0 013 20.5z"/></svg>
+                    <h4>Android</h4>
+                    <p>Google Play</p>
+                </a>
             </div>
 
             <h3>Chrome Installation</h3>