perf: bundle flag icons instead of blocking on a CDN (#99)

index.html loaded two jsdelivr stylesheets synchronously in <head>, which
delayed first paint and failed outright on networks where the CDN is
unreachable. Drop font-logos (unused in src/) and bundle flag-icons
locally so Vite hashes the sprites and serves them from our own origin
with immutable cache headers.

index.html

@@ -72,8 +72,6 @@
     <meta name="apple-mobile-web-app-title" content="Nezha Monitoring" />
     <link rel="apple-touch-icon" href="/android-chrome-192x192.png" />
 
-    <link rel="stylesheet" href="https://fastly.jsdelivr.net/gh/lipis/flag-icons@7.0.0/css/flag-icons.min.css" />
-    <link rel="stylesheet" href="https://fastly.jsdelivr.net/npm/font-logos@1/assets/font-logos.css" />
   </head>
   <body>
     <div id="root"></div>