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.
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