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/index.php

@@ -219,7 +219,7 @@ $today = date('Y-m-d');
                         (empty($ann['end_date']) || $ann['end_date'] >= $today);
                     ?>
                     <tr>
-                      <td>
+                      <td data-label="Status">
                         <?php if ($isCurrentlyActive): ?>
                           <span class="status-badge active">● Live</span>
                         <?php elseif ($ann['is_active']): ?>
@@ -228,23 +228,23 @@ $today = date('Y-m-d');
                           <span class="status-badge inactive">○ Off</span>
                         <?php endif; ?>
                       </td>
-                      <td>
+                      <td data-label="Headline">
                         <strong><?= htmlspecialchars($ann['headline']) ?></strong>
                         <div class="muted" style="font-size: 0.82rem; margin-top: 0.2rem; max-width: 32ch; white-space: nowrap; overflow: hidden; text-overflow: ellipsis;">
                           <?= htmlspecialchars($ann['message']) ?>
                         </div>
                       </td>
-                      <td>
+                      <td data-label="Type">
                         <span class="eyebrow"><?= htmlspecialchars($ann['type']) ?></span>
                       </td>
-                      <td class="font-mono" style="font-size: 0.8rem; color: var(--ink-muted);">
+                      <td class="font-mono" data-label="Schedule" style="font-size: 0.8rem; color: var(--ink-muted);">
                         <?php if (!empty($ann['start_date']) || !empty($ann['end_date'])): ?>
                           <?= htmlspecialchars($ann['start_date'] ?: 'Now') ?> → <?= htmlspecialchars($ann['end_date'] ?: 'Ongoing') ?>
                         <?php else: ?>
                           Always (while on)
                         <?php endif; ?>
                       </td>
-                      <td style="font-size: 0.82rem; color: var(--ink-muted);">
+                      <td data-label="Placements" style="font-size: 0.82rem; color: var(--ink-muted);">
                         <?php
                           $locs = [];
                           if (!empty($ann['show_banner'])) $locs[] = 'Top Banner';
@@ -252,7 +252,7 @@ $today = date('Y-m-d');
                           echo implode(', ', $locs) ?: 'None';
                         ?>
                       </td>
-                      <td>
+                      <td data-label="Actions">
                         <div class="admin-table-actions">
                           <a href="index.php?edit=<?= (int)$ann['id'] ?>" class="btn btn-outline btn-sm">Edit</a>