fix(pos2): wait for branch before fetching menu + add left category sidebar

Race fix: orderBranchId now returns `undefined` (not null) while the /branches
query is in flight. usePos2Menu treats undefined as "not yet determined" and
skips the fetch, preventing getBranchMenu(cafeId, null) → empty array.
Once branchesFetched=true, orderBranchId resolves to the correct branchId
(or null for café-wide fallback).

Layout: desktop order screen now shows a left vertical category sidebar
(116 px, md+) instead of horizontal chips, giving the classic POS sidebar
feel. Horizontal chips kept for mobile (<md). Menu grid columns adjusted.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/dashboard/src/lib/pos2/use-pos2.ts

@@ -22,7 +22,13 @@ export function usePos2Categories(cafeId?: string | null) {
 }
 
 /** Branch-scoped menu (effective prices) when a branch is selected; otherwise the
- *  café-wide menu. Both normalize to MenuItem so the cart store can consume them. */
+ *  café-wide menu. Both normalize to MenuItem so the cart store can consume them.
+ *
+ *  Pass `branchId = undefined` (not null) while still determining which branch to
+ *  use — the query will pause until branchId is resolved. Once resolved:
+ *    • string  → branch-scoped menu via getBranchMenu
+ *    • null    → café-wide fallback via /menu/items
+ */
 export function usePos2Menu(cafeId?: string | null, branchId?: string | null) {
   return useQuery({
     queryKey: ["pos2-menu", cafeId, branchId ?? "cafe"],
@@ -33,7 +39,8 @@ export function usePos2Menu(cafeId?: string | null, branchId?: string | null) {
       }
       return apiGet<MenuItem[]>(`/api/cafes/${cafeId}/menu/items`);
     },
-    enabled: !!cafeId,
+    // branchId === undefined means "still determining" — don't fire yet
+    enabled: !!cafeId && branchId !== undefined,
     staleTime: 30_000,
   });
 }