Feat: add PWA/SEO meta, social cards, and JSON-LD to index.html

* Add Apple Touch Icon and iOS PWA meta (`apple-mobile-web-app-capable`, title)
* Declare `application-name` and `color-scheme` for better theming
* Add Open Graph and Twitter Card tags (title, description, images, locale, site name)
* Include standard HTML `description` meta
* Add author/creator metadata plus `og:see_also` and `rel="me"` links
* Embed Schema.org `WebApplication` JSON-LD (name, urls, description, category, image, author/publisher)
* Minor head cleanup to centralize share preview data and attribution

Author: alisolphp

public/images/og-1200x600.jpg

@@ -6,6 +6,71 @@
     <title>EchoTalk | Offline Shadowing Practice</title>
     <meta name="theme-color" content="#43b6fd">
     <link rel="icon" type="image/png" sizes="32x32" href="icons/icon-512.png">
+    <link rel="apple-touch-icon" href="icons/icon-192.png">
+
+    <meta name="application-name" content="EchoTalk">
+    <meta name="apple-mobile-web-app-capable" content="yes">
+    <meta name="apple-mobile-web-app-title" content="EchoTalk">
+    <meta name="color-scheme" content="dark light">
+
+    <meta property="og:title" content="EchoTalk | Offline Shadowing Practice">
+    <meta property="og:description" content="EchoTalk is a privacy-first offline shadowing app (PWA) for pronunciation and fluency practice.">
+    <meta name="description" content="EchoTalk is a privacy-first offline shadowing app (PWA) for pronunciation and fluency practice.">
+    <meta property="og:type" content="website">
+    <meta property="og:site_name" content="EchoTalk">
+    <meta property="og:locale" content="en_US">
+    <meta property="og:image" content="images/og-1200x600.jpg">
+    <meta property="og:image:width" content="1200">
+    <meta property="og:image:height" content="600">
+    <meta property="og:image:alt" content="EchoTalk — Offline Shadowing Practice">
+    <meta property="og:image" content="images/og-720x720.jpg">
+    <meta property="og:image:width" content="720">
+    <meta property="og:image:height" content="720">
+    <meta property="og:image:alt" content="EchoTalk — Offline Shadowing Practice (square)">
+
+    <meta name="twitter:card" content="summary_large_image">
+    <meta name="twitter:title" content="EchoTalk | Offline Shadowing Practice">
+    <meta name="twitter:description" content="Privacy-first offline shadowing app (PWA) for pronunciation and fluency.">
+    <meta name="twitter:image" content="images/og-1200x600.jpg">
+    <meta name="twitter:image:alt" content="EchoTalk — Offline Shadowing Practice">
+
+    <meta name="author" content="Ali Sol">
+    <meta name="creator" content="Ali Sol">
+    <meta property="og:see_also" content="https://alisol.ir/">
+    <meta property="og:see_also" content="https://github.com/alisolphp/">
+    <link rel="me" href="https://alisol.ir/">
+    <link rel="me" href="https://github.com/alisolphp/">
+    <link rel="me" href="https://www.linkedin.com/in/alisolphp/">
+
+    <script type="application/ld+json">
+        {
+            "@context": "https://schema.org",
+            "@type": "WebApplication",
+            "name": "EchoTalk",
+            "url": ["https://alisolphp.github.io/EchoTalk/","https://alisol.ir/EchoTalk/"],
+            "description": "EchoTalk is a privacy-first offline shadowing app (PWA) for pronunciation and fluency practice.",
+            "applicationCategory": "EducationalApplication",
+            "operatingSystem": "Web",
+            "isAccessibleForFree": true,
+            "image": "/EchoTalk/images/og-1200x600.jpg",
+            "author": {
+                "@type": "Person",
+                "name": "Ali Sol",
+                "url": "https://alisol.ir/",
+                "sameAs": [
+                    "https://alisol.ir/",
+                    "https://github.com/alisolphp/",
+                    "https://www.linkedin.com/in/alisolphp/"
+                ]
+            },
+            "publisher": {
+                "@type": "Person",
+                "name": "Ali Sol",
+                "url": "https://alisol.ir/"
+            }
+        }
+    </script>
+
 </head>
 <body class="d-flex flex-column h-100">