style(dashboard): implement retro terminal gruvbox aesthetic

This commit overhauls the dashboard styling to achieve a 'retro terminal' look matching the Gruvbox color palette. Changes include switching the global font to JetBrains Mono/Fira Code, removing rounded corners on all major elements (cards, buttons, inputs, modals, badges), and replacing modern soft shadows with sharp, hard-offset block shadows.

static/index.html

@@ -4,11 +4,11 @@
     <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=9">
+    <link rel="stylesheet" href="/css/dashboard.css?v=11">
     <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">
-    <link href="https://fonts.googleapis.com/css2?family=Inter:wght@300;400;500;600;700&display=swap" rel="stylesheet">
+    <link href="https://fonts.googleapis.com/css2?family=Fira+Code:wght@300;400;500;600;700&family=JetBrains+Mono:wght@400;700&display=swap" rel="stylesheet">
     <script src="https://cdn.jsdelivr.net/npm/chart.js"></script>
     <script src="https://cdn.jsdelivr.net/npm/luxon@3.4.4/build/global/luxon.min.js"></script>
 </head>
@@ -17,8 +17,7 @@
     <div id="login-screen" class="login-container">
         <div class="login-card">
             <div class="login-header">
-                <img src="img/logo-full.png" alt="LLM Proxy Logo" class="login-logo" onerror="this.style.display='none'; this.nextElementSibling.style.display='block';">
-                <i class="fas fa-robot login-logo-fallback" style="display: none;"></i>
+                <i class="fas fa-terminal login-logo-fallback"></i>
                 <h1>LLM Proxy Gateway</h1>
                 <p class="login-subtitle">Admin Dashboard</p>
             </div>