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.module.scss

@@ -11,10 +11,46 @@
 }
 
 .empty {
-  color: $color-text-muted;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: $space-1;
+  padding: $space-5 $space-3;
+  text-align: center;
+  border: 1px dashed $color-border;
+  border-radius: $radius;
+  background: $color-surface;
 
-  a {
-    color: $color-primary;
+  svg {
+    width: 2.5rem;
+    height: 2.5rem;
+    margin-bottom: $space-2;
+    color: $color-text-muted;
+  }
+}
+
+.emptyTitle {
+  margin: 0;
+  font-size: 1.1rem;
+  font-weight: 600;
+}
+
+.emptyText {
+  margin: 0 0 $space-3;
+  color: $color-text-muted;
+}
+
+.emptyCta {
+  padding: $space-2 $space-4;
+  border-radius: $radius;
+  background: $color-primary;
+  color: $color-primary-contrast;
+  font-weight: 600;
+  text-decoration: none;
+  transition: opacity 0.15s ease;
+
+  &:hover {
+    opacity: 0.9;
   }
 }