refactor(catalog)!: decouple materials from categories (v2.7)

Materials are now global and category-free. The category lives only on the supplier (suppliers.category_id); every category-facing query goes through the supplier edge instead of the material row.

Schema (migration.sql + init_db DDL):
- drop the materials.category_id column, its FK, and the v2.5 first-link-wins automation (fn_derive_material_category + trg_supplier_materials_derive_category) — the DDL runs idempotently at API startup

Backend:
- GET /materials?category_id=… now filters via EXISTS(supplier_materials ⋈ suppliers.category_id)
- quote category filters (quotes list) use the quote's supplier category
- /pm/on-the-way derives the display category from the supplier of the quote
- supplier_materials links no longer derive/return a material category
- expand_service: material expand=category resolves to null (kept for contract compatibility)
- material create/update no longer validates or stores category_id; seed.py aligned

Frontend:
- material creation no longer sends category_id
- formatMaterialTitle reworked to a strict order with « . » separator: [Brand] . [Material] . [Spec 1] . [Spec 2], brand deduped when the stored name already starts with it; specs separator switched from « · » to « . »

Validated: python compile, api + frontend image builds (tsc + vite) clean; branch cut from the tip of prod_v4 (= origin/production, 5a9ef40 — the team's merge made them identical).

backend/app/services/supplier_material_service.py

@@ -1,10 +1,6 @@
-"""Supplier ↔ material link service (v2.5) — the direct edge of the new flow.
+"""Supplier ↔ material link service (v2.6/v2.7) — the direct edge of the catalog flow.
 
-Linking a material to a supplier derives materials.category_id from the
-supplier's category when the material has none (first-link-wins). The same
-rule is enforced by the DB trigger trg_supplier_materials_derive_category;
-this service applies it in the application layer so the ORM state is
-consistent inside the same transaction.
+Suppliers can supply any global material across categories.
 """
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -17,7 +13,7 @@ from app.utils.errors import NotFoundError, ValidationError
 async def link_supplier_material(
     db: AsyncSession, supplier_id, material_id, is_active: bool = True
 ) -> SupplierMaterial:
-    """Upsert a supplier<->material link (idempotent) + derive category (first-link-wins).
+    """Upsert a supplier<->material link (idempotent).
 
     Caller is responsible for the surrounding transaction.
     """
@@ -42,15 +38,11 @@ async def link_supplier_material(
     link = SupplierMaterial(supplier_id=supplier_id, material_id=material_id, is_active=is_active)
     db.add(link)
     await db.flush()
-
-    # First-link-wins: only when the material has no category yet.
-    if material.category_id is None:
-        material.category_id = supplier.category_id
     return link
 
 
 async def unlink_supplier_material(db: AsyncSession, supplier_id, material_id) -> None:
-    """Remove the link. The material's category is NEVER auto-cleared (documented rule)."""
+    """Remove the link."""
     link = await db.scalar(
         select(SupplierMaterial).where(
             SupplierMaterial.supplier_id == supplier_id,