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/admin-sidebar.tsx

@@ -51,7 +51,7 @@ export function AdminSidebar({ className }: AdminSidebarProps) {
     >
       <nav className="space-y-1">
         {navItems.map((item) => {
-          const isActive = pathname === item.href || (item.href !== '/admin' && pathname?.startsWith(item.href))
+          const isActive = pathname === item.href || (item.href !== '/admin' && pathname?.startsWith(item.href + '/'))
 
           return (
             // <a> instead of <Link>: avoids Next.js RSC navigation transitions