fix(insights): a11y + UX Pro Max audit — accessible list view, reduced-motion, focus, lazy-load

Accessibility (CRITIQUE per UI/UX Pro Max skill):
- NetworkGraph Accessibility Grade D → added accessible List view alternative
  (toggle Graph/List with cluster→notes table, keyboard navigable)
- aria-label text summary on graph container for screen readers
- role=button + tabIndex + onKeyDown on bridge note cards (keyboard accessible)
- focus-visible:ring on all interactive cards (isolated clusters, bridges, list items)

UX (HIGH):
- prefers-reduced-motion: whileHover disabled when user prefers reduced motion
- cursor-pointer verified + focus-visible:ring-ochre on all clickable cards
- Mobile sidebar: hamburger Menu button in header (dispatches open-mobile-sidebar)

Performance (MEDIUM):
- NetworkGraph lazy-loaded via next/dynamic (D3 ~200KB deferred, ssr:false)
- Loading spinner shown while D3 chunk loads

i18n:
- listView, graphAriaLabel, listAriaLabel added to 15 locales

memento-note/locales/fr.json

@@ -3536,7 +3536,10 @@
       "scheduledCron": "CRON planifié",
       "lastSync": "Dernière synchro"
     },
-    "resetFocus": "Réinitialiser focus"
+    "resetFocus": "Réinitialiser focus",
+    "listView": "Liste",
+    "graphAriaLabel": "Réseau sémantique : {clusters} thèmes, {notes} notes, {bridges} notes-ponts. Basculez en vue Liste pour une navigation accessible.",
+    "listAriaLabel": "Liste accessible des clusters avec notes et connexions ponts"
   },
   "consent": {
     "banner": {