fix(ux-audit): landing next/image + confirm AlertDialog + search-modal i18n

Landing page:
- <img> → next/image avec width/height (CLS fix)
- padding nav responsive px-4 sm:px-8

note-card:
- confirm() natif → AlertDialog (non-bloquant, stylable)
- showLeaveDialog state + dialog component

search-modal (20 chaînes FR → i18n):
- useLanguage ajouté
- 20 strings FR hardcoded → t('searchModal.*')
- Clés ajoutées dans en.json + fr.json

memento-note/components/landing-page.tsx

@@ -7,6 +7,7 @@ import {
   Box
 } from 'lucide-react'
 import { useRouter } from 'next/navigation'
+import Image from 'next/image'
 import { useLanguage } from '@/lib/i18n'
 import { useState } from 'react'
 
@@ -44,7 +45,7 @@ export function LandingPage() {
   return (
     <div className="min-h-screen bg-paper text-ink font-sans selection:bg-ochre/30 selection:text-ink">
       {/* Navigation */}
-      <nav className="fixed top-0 left-0 right-0 z-[100] bg-paper/80 backdrop-blur-md border-b border-border px-8 py-4 flex items-center justify-between">
+      <nav className="fixed top-0 left-0 right-0 z-[100] bg-paper/80 backdrop-blur-md border-b border-border px-4 sm:px-8 py-4 flex items-center justify-between">
         <div className="flex items-center gap-3">
           <div className="w-10 h-10 bg-ink flex items-center justify-center rounded-xl shadow-lg rotate-3 group hover:rotate-0 transition-transform cursor-pointer">
             <span className="text-paper font-serif text-2xl font-bold">M</span>
@@ -104,7 +105,7 @@ export function LandingPage() {
           {/* App Preview Mockup */}
           <motion.div initial={{ opacity: 0, y: 100 }} animate={{ opacity: 1, y: 0 }} transition={{ duration: 1, delay: 0.2, ease: [0.23, 1, 0.32, 1] }} className="mt-24 relative">
             <div className="relative mx-auto max-w-5xl aspect-[16/10] bg-white rounded-[32px] shadow-[0_40px_100px_-20px_rgba(0,0,0,0.15)] border border-border p-4 overflow-hidden group">
-              <img src="/images/workspace-hero.jpg" alt="Memento Workspace" className="w-full h-full object-cover rounded-2xl filter saturate-[0.8]" />
+              <Image src="/images/workspace-hero.jpg" alt="Memento Workspace" width={1200} height={750} className="w-full h-full object-cover rounded-2xl filter saturate-[0.8]" priority />
               <div className="absolute inset-0 bg-ink/10 group-hover:bg-ink/0 transition-colors duration-500" />
 
               <div className="absolute top-10 right-10 w-64 bg-paper/90 backdrop-blur-xl border border-border p-6 rounded-2xl shadow-2xl">