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.js

@@ -197,10 +197,10 @@ function insertUserRow(user) {
   // HTML string — escapeHtml() only escapes for text-content positions,
   // not attribute values, so it can't safely build an attribute string.
   tr.innerHTML = `
-    <td><strong>${escapeHtml(user.username)}</strong></td>
-    <td>${escapeHtml(user.role.charAt(0).toUpperCase() + user.role.slice(1))}</td>
-    <td class="font-mono item-sort-cell">${escapeHtml(user.created_at)}</td>
-    <td><div class="admin-table-actions">
+    <td data-label="Username"><strong>${escapeHtml(user.username)}</strong></td>
+    <td data-label="Role">${escapeHtml(user.role.charAt(0).toUpperCase() + user.role.slice(1))}</td>
+    <td class="font-mono item-sort-cell" data-label="Created">${escapeHtml(user.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="${user.id}">Change Password</button>
       <button type="button" class="btn btn-danger btn-sm btn-delete-user" data-id="${user.id}">Delete</button>
     </div></td>