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/SupplierDetailPage.tsx

@@ -152,12 +152,12 @@ export function SupplierDetailPage() {
               پیشنهاد جدید
             </Link>
             {firstCat ? (
-              <Link to={`/topics/${encodeURIComponent(topicName)}/${encodeURIComponent(firstCat)}`} className="btn-line-file no-underline">
+              <Link to={`/suppliers/category/${encodeURIComponent(firstCat)}`} className="btn-line-file no-underline">
                 ‹ بازگشت به دسته
               </Link>
             ) : (
-              <Link to={`/topics/${encodeURIComponent(topicName)}`} className="btn-line-file no-underline">
-                ‹ بازگشت به موضوع
+              <Link to="/suppliers" className="btn-line-file no-underline">
+                ‹ بازگشت به دسته‌بندی‌ها
               </Link>
             )}
           </div>