feat: unified Data Import page — merge Dispo + Skill imports

New /admin/imports page with tabs:
- "Dispo Import" tab: renders DispoImportClient (lazy-loaded)
- "Skill Matrix" tab: renders BatchSkillImport (lazy-loaded)
- Tab state via ?tab= URL param

Routing:
- /admin/dispo-imports → redirects to /admin/imports?tab=dispo
- /admin/skill-import → redirects to /admin/imports?tab=skills
- /admin/dispo-imports/[batchId] detail routes still work

Sidebar: single "Data Import" link under ACN-Orga (was 2 links)

Co-Authored-By: claude-flow <ruv@ruv.net>

apps/web/src/components/layout/AppShell.tsx

@@ -182,8 +182,7 @@ const adminNavEntries: AdminEntry[] = [
       { href: "/admin/org-units", label: "Org Units", icon: <AdminIcon /> },
       { href: "/admin/utilization-categories", label: "Util. Categories", icon: <AdminIcon /> },
       { href: "/admin/management-levels", label: "Mgmt Levels", icon: <AdminIcon /> },
-      { href: "/admin/dispo-imports", label: "Dispo Import", icon: <AdminIcon /> },
-      { href: "/admin/skill-import", label: "Skill Import", icon: <AdminIcon /> },
+      { href: "/admin/imports", label: "Data Import", icon: <AdminIcon /> },
     ],
   },
   { href: "/admin/calculation-rules", label: "Calc. Rules", icon: <AdminIcon /> },