feat(article): implement articlesupplier editor tab

- ApiClient: add getSuppliers, addSupplier, updateSupplier,
  orderSuppliers, deleteSupplier on article namespace
- api/index.js: expose same methods in legacy http client
- StorageManager: add #supplierWatchers Map and watchArticleSuppliers()
  backed by article-<sku> SSE signal; wired up as article.supplierWatch
- ArticleTileTab: pass article prop to ArticleSupplierTab
- ArticleSupplierDialog (new): dialog to assign / edit a supplier link
  (supplier autocomplete, Lief.-SKU, VPE, stock, EK Netto + MwSt.)
- ArticleSupplierTab: full supplier management table with reordering
  (up/down arrows → PUT /supplier/order), inline edit and delete,
  stock badge, purchase price display and empty state

src/api/ApiClient.js

@@ -316,6 +316,20 @@ export class ApiClient {
       /** PUT /api/article/{sku}/categories */
       setCategories: (sku, body) =>
         this.#json('PUT', `/api/article/${enc(sku)}/categories`, { body }),
+      /** GET /api/article/{sku}/supplier */
+      getSuppliers: (sku) => this.#json('GET', `/api/article/${enc(sku)}/supplier`),
+      /** POST /api/article/{sku}/supplier */
+      addSupplier: (sku, body) =>
+        this.#json('POST', `/api/article/${enc(sku)}/supplier`, { body }),
+      /** PATCH /api/article/{sku}/supplier/{id} */
+      updateSupplier: (sku, id, body) =>
+        this.#json('PATCH', `/api/article/${enc(sku)}/supplier/${enc(id)}`, { body }),
+      /** PUT /api/article/{sku}/supplier/order */
+      orderSuppliers: (sku, body) =>
+        this.#json('PUT', `/api/article/${enc(sku)}/supplier/order`, { body }),
+      /** DELETE /api/article/{sku}/supplier/{id} */
+      deleteSupplier: (sku, id) =>
+        this.#empty('DELETE', `/api/article/${enc(sku)}/supplier/${enc(id)}`),
     };
 
     this.posterminal = {