feat(units): global-material conversion workspace (no category landing)

- «ضرایب تبدیل» opens directly on the global materials list (no topic/category
  page); sidebar accordion: unique base materials (brand/spec prefixes
  stripped) with expandable spec variants; search toolbar filters live.
- Detail binds to the selected material: reference unit (editable combobox
  over global units, changes canonical_unit_id), conversion factors from
  material_units incl. optional brand scope (restored برند field), add/edit/
  delete with live preview (canonical-first wording), unassigned-units
  warning section, global standard conversions kept.
- Self-referencing children excluded (only base+spec variants listed);
  groups without variants selectable via their representative material.
- New lib/units.ts uniqueUnits(): unit dropdowns show each name once,
  preferring the canonical coded row (kg/piece) over duplicate custom rows
  with identical Persian names.

frontend/src/lib/units.ts

@@ -0,0 +1,27 @@
+import { normalize } from "./normalize";
+
+export interface UnitLike {
+  id: number;
+  code: string;
+  name_fa: string | null;
+}
+
+export function unitDisplayName(u: UnitLike): string {
+  return u.name_fa || u.code;
+}
+
+// واحدها با «نام نمایشی» یکتا می‌شوند: وقتی دو ردیف نام فارسی یکسان دارند
+// (مثلاً ردیف سفارشی «عدد» و ردیف پایه‌ی piece/عدد)، ردیفی که کد استاندارد
+// دارد (code ≠ نام فارسی، مثل kg یا piece) ارجح است.
+export function uniqueUnits(units: UnitLike[]): UnitLike[] {
+  const best = new Map<string, UnitLike>();
+  units.forEach((u) => {
+    const display = unitDisplayName(u);
+    const key = normalize(display);
+    if (!key) return;
+    const cur = best.get(key);
+    const isCanonical = u.code !== display;
+    if (!cur || (isCanonical && cur.code === display)) best.set(key, u);
+  });
+  return [...best.values()].sort((a, b) => unitDisplayName(a).localeCompare(unitDisplayName(b), "fa"));
+}