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/components/orders/OrdersListView.tsx

@@ -9,6 +9,22 @@ type OrdersListViewProps = {
   orders: Array<CustomerOrderSummary>
 }
 
+const OrdersIcon = () => (
+  <svg
+    viewBox="0 0 24 24"
+    fill="none"
+    stroke="currentColor"
+    strokeWidth="1.5"
+    strokeLinecap="round"
+    strokeLinejoin="round"
+    aria-hidden="true"
+  >
+    <path d="M9 3h6l1 3H8l1-3Z" />
+    <path d="M4 6h16l-1.2 13.2A2 2 0 0 1 16.81 21H7.19a2 2 0 0 1-2-1.8L4 6Z" />
+    <path d="M9 11h6" />
+  </svg>
+)
+
 const formatItems = (items: CustomerOrderSummary['items']) =>
   items
     .map(({ menuItemName, quantity }) =>
@@ -20,10 +36,16 @@ export const OrdersListView = ({ orders }: OrdersListViewProps) => (
   <section className={styles.page}>
     <h1 className={styles.heading}>Your orders</h1>
     {orders.length === 0 ? (
-      <p className={styles.empty}>
-        You haven't placed an order yet — <Link to="/">browse the menu</Link>{' '}
-        to get started.
-      </p>
+      <div className={styles.empty}>
+        <OrdersIcon />
+        <p className={styles.emptyTitle}>No orders yet</p>
+        <p className={styles.emptyText}>
+          Your placed orders will show up here.
+        </p>
+        <Link to="/" className={styles.emptyCta}>
+          Browse the menu
+        </Link>
+      </div>
     ) : (
       <ul className={styles.orders}>
         {orders.map((order) => (