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/routes/admin/menu/new.tsx

@@ -0,0 +1,29 @@
+import { createFileRoute, useNavigate } from '@tanstack/react-router'
+
+import { MenuItemForm } from '#/components/admin/MenuItemForm.tsx'
+import { withAdminGuard } from '#/lib/admin-guard.ts'
+import { createMenuItem, getMachineTagOptions } from '#/server/menu-admin.ts'
+
+const NewMenuItemRoute = () => {
+  const machineTags = Route.useLoaderData()
+  const navigate = useNavigate()
+
+  return (
+    <section>
+      <h1>Add menu item</h1>
+      <MenuItemForm
+        machineTags={machineTags}
+        submitLabel="Create item"
+        onSubmit={async (input) => {
+          await createMenuItem({ data: input })
+          await navigate({ to: '/admin/menu', viewTransition: false })
+        }}
+      />
+    </section>
+  )
+}
+
+export const Route = createFileRoute('/admin/menu/new')({
+  loader: () => withAdminGuard(() => getMachineTagOptions()),
+  component: NewMenuItemRoute,
+})