style(dashboard): fix login form floating labels rendering

This commit corrects the CSS for the login form's floating labels. Previously, the label floated too high and had a solid background that contrasted poorly with the input box. The label now floats exactly on the border line and uses a linear-gradient to seamlessly blend the card background into the input background, fixing the 'misframed' appearance.

static/index.html

@@ -4,7 +4,7 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>LLM Proxy Gateway - Admin Dashboard</title>
-    <link rel="stylesheet" href="/css/dashboard.css?v=7">
+    <link rel="stylesheet" href="/css/dashboard.css?v=8">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.4.0/css/all.min.css">
     <link rel="icon" href="img/logo-icon.png" type="image/png" sizes="any">
     <link rel="apple-touch-icon" href="img/logo-icon.png">