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/styles/globals.scss

@@ -73,6 +73,18 @@ button {
   ::view-transition-new(site-header) {
     animation: none;
   }
+
+  // Same reasoning as site-header: the admin sidebar nav is identical across
+  // every /admin/* route, so it should stay put across those navigations
+  // instead of fading/translating along with the root snapshot.
+  ::view-transition-group(admin-sidebar) {
+    animation: none;
+  }
+
+  ::view-transition-old(admin-sidebar),
+  ::view-transition-new(admin-sidebar) {
+    animation: none;
+  }
 }
 
 // document.startViewTransition() itself doesn't check this preference —