GEHEGE-RENAME: UI-Begriff Becken → Gehege + Route /gehege + Icon 🏜️

- de.ts: alle nutzer-sichtbaren 'Becken'-Strings → 'Gehege' (nav, fields,
  titles, buttons, validation, delete, hilfe sections)
- AppShell.tsx: route /becken → /gehege, icon 🛁 → 🏜️ (Wüsten-Habitat)
- App.tsx: <Route path="gehege"> + <Route path="becken/*"> redirect (bewahrt
  Bookmarks; BeckenRedirect leitet Unterseiten 1:1 weiter)
- BeckenPage/DetailPage/FormPage: interne Links /becken → /gehege
- HilfePage.tsx: hartcodierte 'Becken'-Prosa → 'Gehege'
- e2e/becken-kontakte.spec.ts: goto('/becken') → goto('/gehege')
Gate: build ✓  eslint ✓  vitest 82/82 ✓  e2e 120/120 ✓

gerbil-manager-web/src/components/AppShell.tsx

@@ -19,7 +19,7 @@ const PRIMARY: NavItem[] = [
   { to: '/genetik', label: de.nav.genetics, icon: '🧬' },
 ]
 const SECONDARY: NavItem[] = [
-  { to: '/becken', label: de.nav.enclosures, icon: '🛁' },
+  { to: '/gehege', label: de.nav.enclosures, icon: '🏜️' },
   { to: '/kontakte', label: de.nav.contacts, icon: '📇' },
   { to: '/abgabe', label: de.nav.forSale, icon: '🏡' },
   // INBOX-1: Anfragen-Posteingang