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/locales/en.json

@@ -3932,5 +3932,27 @@
     "openSite": "Open site",
     "updateSite": "Update site",
     "unpublish": "Unpublish site"
+  },
+  "searchModal": {
+    "search_ariaLabel": "Search",
+    "search_placeholder": "Search across all your notes…",
+    "search_caseSensitive": "Case sensitive",
+    "search_regexMode": "Regex mode",
+    "search_trashIncluded": "Trash included",
+    "search_openInEditor": "Open in editor",
+    "search_title": "Memento Search",
+    "search_favorites": "Favorites:",
+    "search_searching": "Searching…",
+    "search_noResults": "No results",
+    "search_typeToSearch": "Type to search",
+    "search_aiAnalysis": "Analyzing…",
+    "search_aiResponse": "AI Response",
+    "search_resultsSummary": "Results summary…",
+    "search_documentPreview": "Document preview",
+    "search_noMatch": "No note matches.",
+    "search_typeForResults": "Type to get instant results.",
+    "search_hintNavigate": "navigate",
+    "search_hintOpen": "open",
+    "search_hintClose": "close"
   }
 }