fix: comprehensive i18n — replace hardcoded French/English strings with t() calls

Replaced ~100+ hardcoded French and English text strings across 30+ components
with proper i18n t() calls. Added 57 new translation keys to all 15 locale files
(ar, de, en, es, fa, fr, hi, it, ja, ko, nl, pl, pt, ru, zh).

Key changes:
- contextual-ai-chat.tsx: 30 French strings → t() (actions, toasts, labels, placeholders)
- ai-chat.tsx: 15 French/English strings → t() (header, tabs, welcome, insights, history)
- note-inline-editor.tsx: 20 French fallbacks removed (toolbar, save status, checklist)
- lab-skeleton.tsx: French loading text → t()
- admin-header.tsx, header.tsx, editor-connections-section.tsx: French fallbacks removed
- New AI chat component, agent cards, sidebar, settings panel i18n cleanup

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/components/agents/agent-templates.tsx

@@ -102,7 +102,7 @@ export function AgentTemplates({ onInstalled, existingAgentNames }: AgentTemplat
 
   return (
     <div>
-      <h3 className="text-sm font-semibold text-slate-500 uppercase tracking-wider mb-3">
+      <h3 className="text-xs font-semibold text-muted-foreground uppercase tracking-wider mb-3">
         {t('agents.templates.title')}
       </h3>
       <div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-3">
@@ -115,15 +115,15 @@ export function AgentTemplates({ onInstalled, existingAgentNames }: AgentTemplat
           return (
             <div
               key={tpl.id}
-              className="border-2 border-dashed border-slate-200 rounded-xl p-4 hover:border-primary/30 hover:bg-primary/[0.02] transition-all group"
+              className="border-2 border-dashed border-slate-200 dark:border-slate-700 rounded-xl p-4 hover:border-primary/30 hover:bg-primary/[0.02] transition-all group"
             >
               <div className="flex items-center gap-2.5 mb-2">
                 <div className={`p-1.5 rounded-lg ${typeColors[tpl.type]}`}>
                   <Icon className="w-4 h-4" />
                 </div>
-                <h4 className="font-medium text-sm text-slate-700">{t(nameKey)}</h4>
+                <h4 className="font-medium text-sm text-foreground">{t(nameKey)}</h4>
               </div>
-              <p className="text-xs text-slate-400 mb-3 line-clamp-2">{t(descKey)}</p>
+              <p className="text-xs text-muted-foreground mb-3 line-clamp-2">{t(descKey)}</p>
               <button
                 onClick={() => handleInstall(tpl)}
                 disabled={isInstalling}