Add mobile nav toggle, responsive admin tables, and menu reordering

- Public site: replace the always-hidden mobile nav with a hamburger
  toggle (index.php, menu.php, main.js, styles.css)
- Admin: render the Menu/Announcements/Users tables as labeled cards
  below ~760px instead of squeezing descriptions into unreadable
  single-word-per-line columns
- Admin menu: replace the manual "Order" number (which allowed
  duplicates and never reflected in real time) with ↑/↓ buttons that
  reorder items and categories immediately and renumber sort_order
  sequentially on every move, self-healing any stale duplicates

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

admin/users.php

@@ -195,15 +195,15 @@ $users = get_all_users();
             <tbody id="users-tbody">
               <?php foreach ($users as $row): ?>
                 <tr id="user-row-<?= (int) $row['id'] ?>" data-user-id="<?= (int) $row['id'] ?>">
-                  <td>
+                  <td data-label="Username">
                     <strong><?= htmlspecialchars($row['username']) ?></strong>
                     <?php if ((int) $row['id'] === (int) $user['id']): ?>
                       <span class="muted"> (you)</span>
                     <?php endif; ?>
                   </td>
-                  <td><?= htmlspecialchars(ucfirst($row['role'])) ?></td>
-                  <td class="font-mono item-sort-cell"><?= htmlspecialchars($row['created_at']) ?></td>
-                  <td>
+                  <td data-label="Role"><?= htmlspecialchars(ucfirst($row['role'])) ?></td>
+                  <td class="font-mono item-sort-cell" data-label="Created"><?= htmlspecialchars($row['created_at']) ?></td>
+                  <td data-label="Actions">
                     <div class="admin-table-actions">
                       <button type="button" class="btn btn-outline btn-sm btn-change-password"
                         data-id="<?= (int) $row['id'] ?>"