Suppliers: Optional Handle

Suppliers now carry an optional handle — a URL-friendly identifier, unique per company where set. You can set it when creating a supplier (the create dialog suggests a kebab-cased handle from the name and lets you override it) and edit it later from the supplier detail page's Actions menu. Two suppliers in different companies can still share the same handle, and the field is validated to lowercase letters, numbers, and hyphens up to 50 characters. The suppliers list has a Handle column (hidden by default, toggleable from column settings), and the supplier detail page shows the handle inline in the header. Uniqueness is enforced by a partial unique index (company_id, handle where handle is present).