feat(i18n): Phase 1 — migrate 5 critical files from hardcoded text to i18n

47 new i18n keys added across all 13 locales (en, fr, es, de, pt, it,
nl, ru, ja, ko, zh, ar, fa). English and French are fully translated,
remaining locales use French as placeholder.

Files migrated:
- EditGlossaryDialog.tsx (18 strings)
- DeleteGlossaryDialog.tsx (7 strings)
- ProUpgradePrompt.tsx (10 strings)
- WebhookSnippet.tsx (4 strings)
- TranslationModeToggle.tsx (8 strings)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/app/dashboard/glossaries/DeleteGlossaryDialog.tsx

@@ -10,6 +10,7 @@ import {
   DialogTitle,
 } from '@/components/ui/dialog';
 import { Button } from '@/components/ui/button';
+import { useI18n } from '@/lib/i18n';
 
 interface DeleteGlossaryDialogProps {
   open: boolean;
@@ -26,13 +27,14 @@ export function DeleteGlossaryDialog({
   isDeleting,
   glossaryName,
 }: DeleteGlossaryDialogProps) {
+  const { t } = useI18n();
   return (
     <Dialog open={open} onOpenChange={onOpenChange}>
       <DialogContent className="sm:max-w-md">
         <DialogHeader>
-          <DialogTitle>Delete Glossary</DialogTitle>
+          <DialogTitle>{t('glossaries.delete.title')}</DialogTitle>
           <DialogDescription>
-            Are you sure you want to delete this glossary?
+            {t('glossaries.delete.description')}
             {glossaryName && (
               <span className="block mt-1 font-medium text-foreground">
                 "{glossaryName}"
@@ -45,9 +47,9 @@ export function DeleteGlossaryDialog({
           <div className="flex items-start gap-3">
             <AlertTriangle className="h-5 w-5 text-destructive shrink-0 mt-0.5" />
             <div className="space-y-1">
-              <p className="text-sm font-medium text-destructive">This action cannot be undone</p>
+              <p className="text-sm font-medium text-destructive">{t('glossaries.delete.warning')}</p>
               <p className="text-sm text-muted-foreground">
-                All term pairs will be permanently removed.
+                {t('glossaries.delete.warningDesc')}
               </p>
             </div>
           </div>
@@ -59,14 +61,14 @@ export function DeleteGlossaryDialog({
             onClick={() => onOpenChange(false)}
             disabled={isDeleting}
           >
-            Cancel
+            {t('glossaries.delete.cancel')}
           </Button>
-          <Button 
-            variant="destructive" 
+          <Button
+            variant="destructive"
             onClick={onConfirm}
             disabled={isDeleting}
           >
-            {isDeleting ? 'Deleting...' : 'Delete'}
+            {isDeleting ? t('glossaries.delete.deleting') : t('glossaries.delete.deleteBtn')}
           </Button>
         </DialogFooter>
       </DialogContent>