feat: add analytics page with CA TTC comparison by supplier/nomenclature

- pg-ff-client: add pgGetCaByFournisseur() and pgGetCaByNomenclature()
  returning CA TTC per site and month for monthly N vs N-1 comparison
- analytics/page.tsx: server component with pivot logic to reshape raw
  query data into store-separated columns (292/579/total) with evolution %
- analytics/client.tsx: client controls (mode dropdown, month selector)
  and responsive table with color-coded evolution badges
- sidebar: add "Analytics" link to navigation

Features:
- Switch between "Fournisseur" and "Nomenclature" views
- Select any month for comparison vs same month prior year
- Columns split by store: Frouard/Nancy (292), Houdemont (579), Total
- Each store shows: CA this month, CA N-1, evolution %
- Footer totals row matching sum of detail rows
- FR locale currency formatting (€)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/shared/sidebar.tsx

@@ -11,6 +11,7 @@ import { useSession, signOut } from "next-auth/react";
 const NAV_ITEMS = [
     { icon: BarChart2, label: "Dashboard", href: "/dashboard" },
     { icon: LayoutGrid, label: "Grille", href: "/grid" },
+    { icon: BarChart2, label: "Analytics", href: "/analytics" },
     { icon: Camera, label: "Snapshots", href: "/snapshots" },
     { icon: FileDown, label: "Exports", href: "/exports" },
     { icon: Settings, label: "Paramètres", href: "/settings", adminOnly: true },