Add an admin panel: orders dashboard, menu management, kitchen status

Introduces a role-gated /admin section (sidebar layout, requireAdmin
checked both in route loaders and independently in every admin server
function) with three sections:
- Orders: every order across all customers, and an admin-only order
  detail view nested under /admin so the sidebar persists and admin
  access bypasses the guest-token check.
- Menu: create/edit/delete menu items, their options, and their
  preparation steps; deleting an item with past orders is refused
  (protected by the orderItems FK) instead of crashing.
- Kitchen: read-only live view of every machine's busy/idle status and
  per-tag queue depth, with a manual refresh.

Also:
- Adds a `role` field to the user table (Better Auth additionalFields,
  input:false so it can't be self-assigned) and a db:seed-admin script
  to create/promote an admin account.
- Fixes a real bug found while building this: src/server/auth.ts's
  shared session helpers were plain exported functions using a
  server-only import, which the framework couldn't tree-shake out of
  the client bundle once shared across multiple server/*.ts modules —
  this silently broke hydration app-wide. Fixed with createServerOnlyFn.
- Removes the now-fully-unused @tanstack/devtools-vite plugin.
- Disables view transitions for in-admin navigation so the persistent
  sidebar doesn't cross-fade/shift on every click.
- Gives the "your orders" empty state the same visual treatment as the
  basket page's.

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

src/integrations/better-auth/header-user.tsx

@@ -29,6 +29,21 @@ const OrdersIcon = () => (
   </svg>
 )
 
+const AdminIcon = () => (
+  <svg
+    className={styles.menuIcon}
+    viewBox="0 0 24 24"
+    fill="none"
+    stroke="currentColor"
+    strokeWidth="1.75"
+    strokeLinecap="round"
+    strokeLinejoin="round"
+    aria-hidden="true"
+  >
+    <path d="M12 3l7 3v6c0 4.5-3 8-7 9-4-1-7-4.5-7-9V6l7-3Z" />
+  </svg>
+)
+
 const LogOutIcon = () => (
   <svg
     className={styles.menuIcon}
@@ -114,6 +129,16 @@ const BetterAuthHeader = () => {
                 <OrdersIcon />
                 Orders
               </Link>
+              {session.user.role === 'admin' && (
+                <Link
+                  to="/admin"
+                  role="menuitem"
+                  className={styles.menuButton}
+                >
+                  <AdminIcon />
+                  Administration
+                </Link>
+              )}
               <button
                 type="button"
                 role="menuitem"