feat(dashboard): drop the category filter

The category filter is not used: remove it end to end — the toolbar's category
combobox (desktop + mobile drawer), its option list, the active-filter count and
the filterQuotes() clause it drove, plus the now-unused `category` field in the
Filters type. Supplier and status filters are unchanged.

The backend's own quote filtering by category is untouched (it filters on the
quote's supplier category when ?category_id= is passed), so the API keeps its
contract; only the dashboard UI stops offering the filter.

frontend/src/pages/Dashboard/DashboardPage.tsx

@@ -19,7 +19,7 @@ import { AppSwitcherMenu } from "../../components/AppSwitcherMenu";
 import { AcceptModal } from "./AcceptModal";
 import { EditModal } from "./EditModal";
 
-const EMPTY_FILTERS: Filters = { category: "", supplier: "", status: "", search: "" };
+const EMPTY_FILTERS: Filters = { supplier: "", status: "", search: "" };
 
 export function DashboardPage() {
   useStoreData();
@@ -80,12 +80,6 @@ export function DashboardPage() {
     }));
   };
 
-  // دسته‌ها روی تأمین‌کننده هستند (v2.7) و بک‌اند هم فیلتر پیشنهادها را با
-  // دسته‌ی تأمین‌کننده انجام می‌دهد — پس گزینه‌های فیلتر از همان‌جا می‌آید.
-  const categoryOptions = useMemo(
-    () => [...new Set(suppliers.flatMap((s) => s.categories || []).filter(Boolean))].sort((a, b) => a.localeCompare(b, "fa")),
-    [suppliers],
-  );
   const supplierOptions = useMemo(() => suppliers.map((s) => ({ id: s.id, name: s.name })), [suppliers]);
 
   return (
@@ -143,7 +137,6 @@ export function DashboardPage() {
         <FilterToolbar
           filters={filters}
           onChange={setFilters}
-          categories={categoryOptions}
           suppliers={supplierOptions}
           sortDir={sortDir}
           onToggleSort={() => setSortDir((d) => (d === "desc" ? "asc" : "desc"))}