feat: diamond app icon + fix login page vertical centering on mobile

- New diamond gem icon (blue gradient background, white faceted gem)
- Replaces all PWA icons: icon-192.png, icon-512.png, apple-touch-icon.png
- Login page uses icon-192.png as logo
- Fix vertical centering with 100dvh to account for mobile browser chrome

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

frontend/src/pages/Login.tsx

@@ -30,14 +30,12 @@ export default function LoginPage({ onLogin }: Props) {
   }
 
   return (
-    <div className="min-h-screen bg-gradient-to-br from-blue-50 to-indigo-100 flex items-center justify-center p-4">
+    <div className="bg-gradient-to-br from-blue-50 to-indigo-100 flex items-center justify-center p-4" style={{ minHeight: '100dvh' }}>
       <div className="bg-white rounded-2xl shadow-xl w-full max-w-sm p-8">
         {/* Logo */}
         <div className="text-center mb-8">
-          <div className="w-16 h-16 bg-blue-500 rounded-2xl flex items-center justify-center mx-auto mb-4">
-            <svg className="w-9 h-9 text-white" fill="currentColor" viewBox="0 0 24 24">
-              <path d="M20 2H4c-1.1 0-2 .9-2 2v18l4-4h14c1.1 0 2-.9 2-2V4c0-1.1-.9-2-2-2z"/>
-            </svg>
+          <div className="w-20 h-20 mx-auto mb-4">
+            <img src="/icon-192.png" alt="JaniChat" className="w-20 h-20 rounded-2xl shadow-lg" />
           </div>
           <h1 className="text-2xl font-bold text-gray-900">JaniChat</h1>
           <p className="text-gray-500 text-sm mt-1">Войдите в аккаунт</p>