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/dashboard/page.tsx

@@ -4,6 +4,7 @@ import { useEffect, useState } from 'react';
 import { useRouter, useSearchParams } from 'next/navigation';
 import { Loader2 } from 'lucide-react';
 import { useUser } from './useUser';
+import { useI18n } from '@/lib/i18n';
 import { API_BASE } from '@/lib/config';
 
 /**
@@ -18,6 +19,7 @@ export default function DashboardPage() {
   const checkoutSessionId = searchParams.get('session_id');
   const [syncError, setSyncError] = useState<string | null>(null);
   const { refetch } = useUser();
+  const { t } = useI18n();
 
   useEffect(() => {
     if (!checkoutSessionId) {
@@ -42,20 +44,20 @@ export default function DashboardPage() {
         if (!cancelled) {
           if (!res.ok) {
             const errData = await res.json().catch(() => ({}));
-            setSyncError(errData.message || 'Erreur lors de la synchronisation du paiement.');
+            setSyncError(errData.message || t('dashboard.checkoutSyncError'));
           } else {
             await refetch();
             router.replace('/dashboard/translate');
           }
         }
       } catch {
-        if (!cancelled) setSyncError('Erreur réseau. Veuillez rafraîchir la page.');
+        if (!cancelled) setSyncError(t('dashboard.networkRefresh'));
       }
     };
 
     runSync();
     return () => { cancelled = true; };
-  }, [checkoutSessionId, refetch, router]);
+  }, [checkoutSessionId, refetch, router, t]);
 
   if (syncError) {
     return (
@@ -65,7 +67,7 @@ export default function DashboardPage() {
           onClick={() => router.replace('/dashboard/translate')}
           className="text-xs text-muted-foreground underline"
         >
-          Continuer vers la traduction
+          {t('dashboard.continueToTranslate')}
         </button>
       </div>
     );