Add filemtime-based cache busting for styles.css and main.js

Safari caches static assets far more aggressively than Chrome/Firefox
and largely ignores heuristic revalidation, so a new deploy could keep
showing the old CSS/JS indefinitely even on a hard refresh — no
Cache-Control headers were set anywhere to guide it otherwise.

inc/assets.php adds asset_version($path), which appends a ?v=<mtime>
query string derived from the file's last-modified time. Every page
that links styles.css or main.js (index.php, menu.php, and all five
admin pages) now uses it, so the URL changes whenever the file's
contents change and browsers always fetch the new version.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

admin/login.php

@@ -1,5 +1,6 @@
 <?php
 require_once __DIR__ . '/../inc/auth.php';
+require_once __DIR__ . '/../inc/assets.php';
 
 $error = '';
 
@@ -24,7 +25,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST') {
   <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:wght@600&family=Karla:wght@400;500;600&family=IBM+Plex+Mono:wght@400;500&display=swap" rel="stylesheet">
-  <link rel="stylesheet" href="../styles.css">
+  <link rel="stylesheet" href="../styles.css<?= asset_version('styles.css') ?>">
 </head>
 <body class="admin-login-body">
   <div class="login-card">