feat: add admin login page and authentication API

- Created a new login page for admin access at frontend/admin/login/index.html.
- Implemented form submission with error handling for login credentials.
- Updated frontend/index.html to mute the video player and removed unnecessary Alpine.js data binding.
- Enhanced manifest URL resolution in frontend/src/main.ts to include cache-busting parameters.
- Modified vite.config.ts to include the new admin login page in the build process.
- Updated nginx.conf to handle admin login routing, authentication, and API proxying for admin actions.

frontend/admin/login/index.html

@@ -0,0 +1,70 @@
+<!doctype html>
+<html lang="en">
+
+<head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>Linear Void Admin Login</title>
+    <script src="https://cdn.tailwindcss.com"></script>
+    <link rel="stylesheet" href="../../src/style.css" />
+</head>
+
+<body class="min-h-screen bg-slate-950 text-slate-100 flex items-center justify-center px-6">
+    <main class="w-full max-w-md bg-slate-900 border border-slate-800 rounded-[2rem] shadow-2xl p-8">
+        <div class="mb-8">
+            <div class="text-xs uppercase tracking-[0.3em] text-slate-500 mb-3">Linear Void</div>
+            <h1 class="text-4xl font-semibold text-white mb-2">Admin Login</h1>
+            <p class="text-sm text-slate-400">Authenticate to access protected admin controls.</p>
+        </div>
+
+        <form id="login-form" class="space-y-4">
+            <label class="block text-sm">
+                <span class="block text-slate-400 mb-2">Username</span>
+                <input id="username" type="text" class="w-full bg-slate-950 border border-slate-700 rounded-xl px-4 py-3 focus:outline-none focus:border-blue-500" autocomplete="username" required />
+            </label>
+            <label class="block text-sm">
+                <span class="block text-slate-400 mb-2">Password</span>
+                <input id="password" type="password" class="w-full bg-slate-950 border border-slate-700 rounded-xl px-4 py-3 focus:outline-none focus:border-blue-500" autocomplete="current-password" required />
+            </label>
+            <button id="submit" type="submit" class="w-full bg-blue-600 hover:bg-blue-500 px-4 py-3 rounded-xl font-medium text-white">Sign in</button>
+            <div id="status" class="text-sm text-slate-400 min-h-6"></div>
+        </form>
+    </main>
+
+    <script>
+        const form = document.getElementById('login-form');
+        const username = document.getElementById('username');
+        const password = document.getElementById('password');
+        const submit = document.getElementById('submit');
+        const status = document.getElementById('status');
+
+        form.addEventListener('submit', async (event) => {
+            event.preventDefault();
+            submit.disabled = true;
+            status.textContent = 'Signing in...';
+            status.className = 'text-sm text-slate-400 min-h-6';
+
+            try {
+                const response = await fetch('/api/admin/login', {
+                    method: 'POST',
+                    headers: { 'Content-Type': 'application/json' },
+                    body: JSON.stringify({ username: username.value, password: password.value })
+                });
+
+                if (!response.ok) {
+                    throw new Error(`HTTP ${response.status}`);
+                }
+
+                window.location.href = '/admin/';
+            } catch (error) {
+                console.error('Failed to sign in', error);
+                status.textContent = 'Login failed. Check your credentials.';
+                status.className = 'text-sm text-rose-400 min-h-6';
+            } finally {
+                submit.disabled = false;
+            }
+        });
+    </script>
+</body>
+
+</html>