refactor(catalog): reduce the category taxonomy to a supplier label

Categories are kept only as the label each supplier carries (suppliers.category_id
+ the material_categories list it points at). Everything that treated category as
a shared key across materials, inventory, quotes and unit conversions is removed —
including two features that were never populated (0 rows on every volume).

Removed (backend):
- routers category_attributes, category_ref_units, unit_conversions + registration
- ?category_id= filters on /materials, /inventory and /quotes (the /suppliers one
  stays — it filters suppliers by their own label)
- GET /inventory/materials/options: required a category and had no caller
- the material-payload category derivation (expand_service.category_by_material,
  the inventory serializer lookup) and the material category on /pm/on-the-way;
  the wire keys stay, now always null, so the payload shape is unchanged
- startup DDL: DROP the empty category_attributes / category_reference_units /
  unit_conversions tables (idempotent) and their CREATE statements; seed.py
  truncate list updated

Removed (frontend):
- the category-keyed conversion path: refUnitForCategory / findConversion /
  refConverted, the categoryUnits + unitConversions store slices, api clients,
  hooks and the "≈ converted quantity" line on quote detail and project pages
  (it needed rows in those tables to render, and there were none)
- material.supplier_category and its read sites (the edit modal grew one
  yesterday to restore labels; the label lives on the supplier now)
- the inventory page's category chips, the row filter and the «دسته» column,
  plus the lookup's categories fetch and catName helper

Kept: supplier label + its list and picker, /suppliers?category_id=, the global
standard_conversions feature and the units page (material_units conversions).

Verified against a restore of the production dump: startup DDL drops exactly the
three tables, suppliers keep their label (3/3), material_categories still serves
its 8 rows, the removed endpoints 404, and materials/inventory/quotes/units/
standard-conversions all return 200 with quote line items intact.

frontend/src/hooks/useUnits.ts

@@ -1,52 +1,16 @@
 // ============================================================
-// hooks واحدها — واحد مرجع، ضرایب دسته‌ای و ثابت
+// hooks واحدها — تبدیل‌های ثابت جهانی (دسته‌ای‌ها حذف شدند)
 // ============================================================
 
 import { useMutation } from "@tanstack/react-query";
-import type { CategoryUnit, StandardConversion, UnitConversion } from "../lib/types";
+import type { StandardConversion } from "../lib/types";
 import { api } from "../lib/api";
 import { useAppStore } from "../store";
 import { useInvalidateStore } from "./useStoreData";
 
-export function useUnits(): {
-  categoryUnits: CategoryUnit[];
-  unitConversions: UnitConversion[];
-  standardConversions: StandardConversion[];
-} {
-  const categoryUnits = useAppStore((s) => s.categoryUnits);
-  const unitConversions = useAppStore((s) => s.unitConversions);
+export function useUnits(): { standardConversions: StandardConversion[] } {
   const standardConversions = useAppStore((s) => s.standardConversions);
-  return { categoryUnits, unitConversions, standardConversions };
-}
-
-export function useSaveRefUnit() {
-  const setCategoryUnit = useAppStore((s) => s.setCategoryUnit);
-  const invalidate = useInvalidateStore();
-  return useMutation({
-    mutationFn: (cu: CategoryUnit) => api.units.saveCategoryUnit(cu),
-    onMutate: (cu) => setCategoryUnit(cu),
-    onSettled: () => invalidate(),
-  });
-}
-
-export function useSaveConversion() {
-  const setConversion = useAppStore((s) => s.setConversion);
-  const invalidate = useInvalidateStore();
-  return useMutation({
-    mutationFn: (c: UnitConversion) => api.units.saveConversion(c),
-    onMutate: (c) => setConversion(c),
-    onSettled: () => invalidate(),
-  });
-}
-
-export function useDeleteConversion() {
-  const deleteConversion = useAppStore((s) => s.deleteConversion);
-  const invalidate = useInvalidateStore();
-  return useMutation({
-    mutationFn: (id: string) => api.units.deleteConversion(id),
-    onMutate: (id) => deleteConversion(id),
-    onSettled: () => invalidate(),
-  });
+  return { standardConversions };
 }
 
 export function useSaveStandardConversion() {