feat: add a meta description for search results and crawlers

The README's opening pitch, trimmed to Google's ~160-character
snippet length.

index.html

@@ -4,6 +4,10 @@
     <meta charset="UTF-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>EdenText</title>
+    <meta
+      name="description"
+      content="EdenText is a web-based, powerful word processor for everything from quick notes to full-length books. No server, no account — your documents never leave your computer."
+    />
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <link rel="manifest" href="/manifest.webmanifest" />
     <link rel="apple-touch-icon" href="/icon-192.png" />