chore(18-01): remove forecast route, sidebar entry, and revalidatePath calls (CLEAN-01)

- Remove Forecast NAV_ITEMS entry and TrendingUp import from AdminSidebar
- Delete src/app/admin/forecast/page.tsx and empty directory
- Remove revalidatePath("/admin/forecast") from assignOfferToProject, removeProjectOffer, updateProjectOfferTotal

src/components/admin/AdminSidebar.tsx

@@ -8,7 +8,6 @@ import {
   Users,
   FolderOpen,
   Tag,
-  TrendingUp,
   BookOpen,
   Settings,
   LogOut,
@@ -21,7 +20,6 @@ const NAV_ITEMS = [
   { href: "/admin/leads",        label: "Lead",         icon: Zap },
   { href: "/admin/projects",     label: "Progetti",     icon: FolderOpen },
   { href: "/admin/offers",       label: "Offerte",      icon: Tag },
-  { href: "/admin/forecast",     label: "Forecast",     icon: TrendingUp },
   { href: "/admin/catalog",      label: "Catalogo",     icon: BookOpen },
   { href: "/admin/impostazioni", label: "Impostazioni", icon: Settings },
 ];