fix(suppliers): enforce Category → Supplier hierarchy, remove rogue-category bug

Domain model (v2.6): suppliers.category_id NOT NULL (1:N from the 8 fixed
categories), materials are global and link to suppliers via
supplier_materials. This fixes three regressions of the old inverted tree:

- remove the intermediate 'یک متریال را انتخاب کنید' screen: a category
  card now routes straight to that category's supplier list
  (/suppliers/category/:category); TopicPage is deleted and legacy
  /topics/:topic[/:category] URLs redirect to the new hierarchy
- kill the accidental-category bug: api.materials.create silently POSTed
  /material-categories for any unknown 'category' (a material name!), which
  is how the rogue «پریز» category (+ junk material) appeared. Unknown
  categories now fail loudly instead of creating catalog rows (DB cleaned:
  exactly the 8 canonical categories remain)
- fix inverted wording: supplier list is «تأمین‌کننده‌های این دسته» with
  category-first breadcrumbs; the legacy material multi-select on the
  add-supplier form is gone (a supplier gets the page's category)
- supplier details screen logic untouched — only its breadcrumb link
  targets were updated to the new routes

frontend/src/pages/Suppliers/SuppliersPage.tsx

@@ -25,14 +25,14 @@ export function SuppliersPage() {
     <div className="min-h-screen">
       <Header
         title="تأمین‌کننده‌ها"
-        sub="اول موضوع (دسته‌بندی) را انتخاب کنید"
+        sub="اول دسته‌بندی را انتخاب کنید"
         backTo="/"
         backLabel="بازگشت به دفتر"
       />
       <PageLayout>
         <TopicGrid
           topics={topics}
-          onSelect={(topic) => navigate(`/topics/${encodeURIComponent(topic)}`)}
+          onSelect={(topic) => navigate(`/suppliers/category/${encodeURIComponent(topic)}`)}
           showCounts
           counts={counts}
         />