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/translate/LanguageSelector.tsx

@@ -32,6 +32,7 @@ function Combobox({
   placeholder: string;
   onChange: (code: string) => void;
 }) {
+  const { t } = useI18n();
   const [open, setOpen] = useState(false);
   const [query, setQuery] = useState('');
   const ref = useRef<HTMLDivElement>(null);
@@ -81,13 +82,13 @@ function Combobox({
               type="text"
               value={query}
               onChange={e => setQuery(e.target.value)}
-              placeholder="Search..."
+              placeholder={t('langSelector.search')}
               className="w-full bg-transparent px-1 py-1 text-xs outline-none placeholder:text-brand-dark/30 dark:placeholder:text-white/30 text-brand-dark dark:text-white"
             />
           </div>
           <div className="max-h-[160px] overflow-y-auto p-1 mt-1 space-y-0.5">
             {filtered.length === 0 && (
-              <div className="px-3 py-3 text-center text-xs text-brand-dark/40 dark:text-white/40">No results</div>
+              <div className="px-3 py-3 text-center text-xs text-brand-dark/40 dark:text-white/40">{t('langSelector.noResults')}</div>
             )}
             {filtered.map(lang => (
               <button
@@ -143,7 +144,7 @@ export default function LanguageSelector({
     <div className="grid grid-cols-11 gap-2 items-center">
       {/* Source */}
       <div className="col-span-5 relative text-left">
-        <span className="text-[10px] font-bold text-brand-dark/40 dark:text-white/40 uppercase tracking-widest block mb-1">Source</span>
+        <span className="text-[10px] font-bold text-brand-dark/40 dark:text-white/40 uppercase tracking-widest block mb-1">{t('langSelector.source')}</span>
         <Combobox
           value={sourceLang}
           options={languages}
@@ -166,7 +167,7 @@ export default function LanguageSelector({
               ? 'text-brand-dark/50 dark:text-white/50 hover:text-brand-accent hover:bg-brand-muted/50 dark:hover:bg-white/5'
               : 'cursor-not-allowed opacity-30'
           )}
-          title="Inverser"
+          title={t('langSelector.swap')}
         >
           <ArrowRightLeft size={12} />
         </button>
@@ -174,7 +175,7 @@ export default function LanguageSelector({
 
       {/* Target */}
       <div className="col-span-5 relative text-left">
-        <span className="text-[10px] font-bold text-brand-dark/40 dark:text-white/40 uppercase tracking-widest block mb-1">Cible</span>
+        <span className="text-[10px] font-bold text-brand-dark/40 dark:text-white/40 uppercase tracking-widest block mb-1">{t('langSelector.target')}</span>
         <Combobox
           value={targetLang}
           options={languages}