feat: add Open Graph tags, a canonical URL and a 1200x630 preview image

Shared links now render a preview card; the image is the README
screenshot center-cropped to the standard card ratio.

index.html

@@ -8,6 +8,18 @@
       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="canonical" href="https://edentext.app/" />
+    <meta property="og:type" content="website" />
+    <meta property="og:url" content="https://edentext.app/" />
+    <meta property="og:title" content="EdenText — Powerful word processor, just one URL away" />
+    <meta
+      property="og: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."
+    />
+    <meta property="og:image" content="https://edentext.app/og-image.png" />
+    <meta property="og:image:width" content="1200" />
+    <meta property="og:image:height" content="630" />
+    <meta name="twitter:card" content="summary_large_image" />
     <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" />