feat: prevent duplicate glossary presets + fix i18n source warning bug

- Add template_id column to Glossary model (nullable, indexed)
- Backend: return 409 Conflict if user already imported a template
- Frontend: disable preset cards already imported, show 'Imported' badge
- Fix duplicated text in GlossarySelector source warning (hardcoded FR text removed)
- Complete i18n migration for glossaries page and GlossarySelector
- Add glossaries.presets.alreadyImported key in all 13 locales

frontend/src/app/dashboard/translate/GlossarySelector.tsx

@@ -338,7 +338,7 @@ export function GlossarySelector({ sourceLang, targetLang, isPro, mode, glossary
             <div className="flex items-start gap-1.5 p-2 rounded-lg bg-amber-500/10 border border-amber-500/20 text-amber-600 dark:text-amber-400 text-[10px] leading-normal font-medium animate-fade-in">
               <span className="shrink-0 text-amber-500">⚠️</span>
               <span>
-                <strong>{t('translate.glossary.sourceWarning') || 'Attention :'} Ce glossaire utilise la langue source</strong> <strong>{getFlag(selected.source_language)} {selected.source_language.toUpperCase()}</strong>, {t('translate.glossary.sourceWarningBut') || 'mais votre document est configuré en'} <strong>{getFlag(sourceLang)} {sourceLang.toUpperCase()}</strong>.
+                <strong>{t('translate.glossary.sourceWarning')}</strong> <strong>{getFlag(selected.source_language)} {selected.source_language.toUpperCase()}</strong>, {t('translate.glossary.sourceWarningBut')} <strong>{getFlag(sourceLang)} {sourceLang.toUpperCase()}</strong>.
               </span>
             </div>
           )}