Refactor admin pages to use shared head and navigation components

- Created `admin-head.php` for consistent <head> markup across admin pages.
- Created `admin-nav.php` for shared navigation bar, reducing duplication.
- Updated all admin pages (account, hours, index, login, menu, users) to utilize the new components.
- Added security headers to `auth.php` and `index.php` for improved security.
- Introduced `security-headers.php` to manage security headers across the application.
- Moved inline styles from admin pages to SCSS files to comply with Content Security Policy (CSP).
- Normalized file permissions during build process to prevent permission issues on deployment.

inc/admin-head.php

@@ -0,0 +1,23 @@
+<?php
+declare(strict_types=1);
+
+require_once __DIR__ . '/assets.php';
+
+/**
+ * Shared <head> for every admin page. $title is the page's own name; the
+ * site suffix is added here so every tab reads "<Page> | The Old Fitz Admin".
+ */
+function render_admin_head(string $title): void {
+	?>
+<head>
+	<meta charset="UTF-8">
+	<meta name="viewport" content="width=device-width, initial-scale=1">
+	<meta name="robots" content="noindex, nofollow">
+	<title><?= htmlspecialchars($title) ?> | The Old Fitz Admin</title>
+	<link rel="preconnect" href="https://fonts.googleapis.com">
+	<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
+	<link href="https://fonts.googleapis.com/css2?family=Fraunces:ital,opsz,wght@0,9..144,400;0,9..144,600;1,9..144,400&family=Karla:wght@400;500;600;700&family=IBM+Plex+Mono:wght@400;500&display=swap" rel="stylesheet">
+	<link rel="stylesheet" href="../styles.css<?= asset_version('styles.css') ?>">
+</head>
+	<?php
+}