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/api-keys/page.tsx

@@ -197,7 +197,7 @@ export default function ApiKeysPage() {
               {t('apiKeys.sectionTitle')}
             </p>
             <p className="text-sm font-mono text-brand-dark dark:text-white">
-              {keys.length > 0 ? `${keys[0].key_prefix}************************************` : 'No keys generated'}
+              {keys.length > 0 ? `${keys[0].key_prefix}************************************` : t('apiKeys.noKeysGenerated')}
             </p>
           </div>
           <div className="flex gap-3">