i18n: fix missing keys and translate all non-admin frontend strings

- Add 12 missing i18n keys (t() was returning the literal key string) to
  all 13 locales: dashboard.topbar.premiumAccess,
  dashboard.translate.complete.toastOkDesc,
  dashboard.translate.progress.{connectionLost,processingFallback},
  glossaries.card.{term,created}, glossaries.termEditor.{addTerm,maxReached},
  login.google.{connecting,errorFailed,errorGeneric}, login.orContinueWith
- Add 6 FR-drift keys (landing.pricing.{free,enterprise}.{name,desc,cta})
- Add ~120 new i18n keys covering site header/footer, file-uploader,
  checkout success, dashboard pages, translate page, provider selector
  themes, language selector, translation complete, api-keys, services,
  settings, pricing (~1800 new key/locale pairs)
- Wrap hardcoded French/English in components with t() calls
- Convert LLM_THEMES/CLASSIC_THEMES/FALLBACK_PROVIDERS maps from
  hardcoded constants to t()-driven factories
- Admin pages intentionally left untouched per request

Files: 15 components/pages + src/lib/i18n.tsx
Typecheck: passes (tsc --noEmit exit 0)

frontend/src/app/checkout/success/page.tsx

@@ -5,6 +5,7 @@ import { useSearchParams, useRouter } from 'next/navigation';
 import { useQueryClient } from '@tanstack/react-query';
 import { API_BASE } from '@/lib/config';
 import { CheckCircle2, XCircle, RefreshCw } from 'lucide-react';
+import { useI18n } from '@/lib/i18n';
 
 /**
  * /checkout/success
@@ -17,6 +18,7 @@ export default function CheckoutSuccessPage() {
   const router = useRouter();
   const sessionId = searchParams.get('session_id');
   const queryClient = useQueryClient();
+  const { t } = useI18n();
 
   const [status, setStatus] = useState<'syncing' | 'ok' | 'error'>('syncing');
   const [message, setMessage] = useState('');
@@ -43,19 +45,23 @@ export default function CheckoutSuccessPage() {
         try { data = await res.json(); } catch { /* ignore */ }
         if (res.ok) {
           setStatus('ok');
-          setMessage(data.data?.plan ? `Forfait ${data.data.plan} activé !` : 'Abonnement activé !');
+          setMessage(
+            data.data?.plan
+              ? t('checkout.planActivated', { plan: data.data.plan })
+              : t('checkout.subscriptionActivated')
+          );
           queryClient.invalidateQueries({ queryKey: ['user', 'me'] });
           // Redirect after 2s
           setTimeout(() => router.replace('/dashboard/profile?tab=subscription'), 2000);
         } else {
           setStatus('error');
-          setMessage(data.message ?? data.error ?? 'Erreur de synchronisation');
+          setMessage(data.message ?? data.error ?? t('checkout.syncError'));
           setTimeout(() => router.replace('/dashboard/profile?tab=subscription'), 3000);
         }
       })
       .catch(() => {
         setStatus('error');
-        setMessage('Erreur réseau. Votre paiement est confirmé — rechargez votre profil.');
+        setMessage(t('checkout.networkError'));
         setTimeout(() => router.replace('/dashboard/profile?tab=subscription'), 3000);
       });
   // eslint-disable-next-line react-hooks/exhaustive-deps
@@ -67,24 +73,24 @@ export default function CheckoutSuccessPage() {
         {status === 'syncing' && (
           <>
             <RefreshCw className="w-12 h-12 text-brand-accent animate-spin" />
-            <h1 className="text-2xl font-black uppercase tracking-tight">Activation en cours…</h1>
-            <p className="text-sm text-muted-foreground">Nous mettons à jour votre abonnement, veuillez patienter.</p>
+            <h1 className="text-2xl font-black uppercase tracking-tight">{t('checkout.activating')}</h1>
+            <p className="text-sm text-muted-foreground">{t('checkout.activatingDesc')}</p>
           </>
         )}
         {status === 'ok' && (
           <>
             <CheckCircle2 className="w-12 h-12 text-emerald-500" />
-            <h1 className="text-2xl font-black uppercase tracking-tight text-emerald-600">Paiement confirmé !</h1>
+            <h1 className="text-2xl font-black uppercase tracking-tight text-emerald-600">{t('checkout.paymentConfirmed')}</h1>
             <p className="text-sm text-muted-foreground">{message}</p>
-            <p className="text-xs text-muted-foreground">Redirection vers votre profil…</p>
+            <p className="text-xs text-muted-foreground">{t('checkout.redirectingToProfile')}</p>
           </>
         )}
         {status === 'error' && (
           <>
             <XCircle className="w-12 h-12 text-amber-500" />
-            <h1 className="text-2xl font-black uppercase tracking-tight">Paiement reçu</h1>
+            <h1 className="text-2xl font-black uppercase tracking-tight">{t('checkout.paymentReceived')}</h1>
             <p className="text-sm text-muted-foreground">{message}</p>
-            <p className="text-xs text-muted-foreground">Redirection…</p>
+            <p className="text-xs text-muted-foreground">{t('checkout.redirecting')}</p>
           </>
         )}
       </div>