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.

index.php

@@ -1,6 +1,8 @@
 <?php
 require __DIR__ . '/config.php';
 require __DIR__ . '/inc/assets.php';
+require __DIR__ . '/inc/security-headers.php';
+send_security_headers();
 require __DIR__ . '/inc/google-reviews.php';
 require __DIR__ . '/inc/announcements.php';
 require __DIR__ . '/inc/hours.php';