feat(brainstorm): UX fixes — modal confirmation + destination carnet + race fix

UX Pro Max appliqué selon les best practices 2026 (Heptabase, Obsidian, Tana):
- confirmation-dialogs (HIGH): modal de confirmation avant brainstorm
- success-feedback (MEDIUM): toast avec destination du carnet
- nav-state-active (MEDIUM): indication claire de la session active

3 fixes:

1. Modal de confirmation (brainstorm-confirm-dialog.tsx, 181 lignes):
   - Design system respecté: brand-accent, memento-paper, font-memento-serif
   - Aperçu seed en card paper
   - Sélecteur de carnet avec animation d'expansion
   - Bouton 'Annuler' + 'Lancer le brainstorm' avec loader
   - role=dialog, aria-modal, aria-label
   - focus-visible, keyboard nav
   - backdrop-blur-sm + click-to-close
   - Lazy loaded via next/dynamic

2. Race condition fix (brainstorm-page.tsx L111):
   - Auto-select session désactivé si urlSeed présent
   - urlSeed + urlInviteToken court-circuitent le useEffect

3. Toast avec destination (handleConvert + noteCreatedIn key):
   - message: 'Note créée dans [Nom du carnet]'
   - useConvertIdea accepte notebookId optionnel
   - API convert accepte notebookId via zod schema
   - useConvertIdea(mutationFn) accepte string | {ideaId, notebookId}

i18n: 9 nouvelles clés brainstorm.* en EN/FR

memento-note/locales/en.json

@@ -2925,7 +2925,7 @@
     "noSessions": "No brainstorms yet",
     "startOne": "Start one",
     "sessions": "Brainstorms",
-    "seedLabel": "Seed Idea",
+    "seedLabel": "Seed idea",
     "ideaPromptDetailed": "Enter your idea, question, or topic to brainstorm...",
     "brainstormThisIdea": "Brainstorm this idea",
     "startBrainstorm": "Start Brainstorm",
@@ -3068,7 +3068,15 @@
     "legendWave1": "Variations",
     "legendWave2": "Analogies",
     "legendWave3": "Disruptions",
-    "legendConverted": "Converted"
+    "legendConverted": "Converted",
+    "confirmTitle": "Brainstorm this idea",
+    "confirmSubtitle": "AI will expand your idea",
+    "destinationLabel": "Notes will be saved to",
+    "defaultNotebook": "Brainstorm",
+    "noNotebook": "No notebook",
+    "launching": "Launching…",
+    "launch": "Brainstorm",
+    "noteCreatedIn": "Note created in"
   },
   "byokSettings": {
     "title": "Your API keys (BYOK)",

memento-note/locales/fr.json

@@ -3072,7 +3072,15 @@
     "legendWave1": "Variations",
     "legendWave2": "Analogies",
     "legendWave3": "Disruptions",
-    "legendConverted": "Convertie"
+    "legendConverted": "Convertie",
+    "confirmTitle": "Brainstormer cette idée",
+    "confirmSubtitle": "L'IA va développer votre idée",
+    "destinationLabel": "Les notes seront sauvegardées dans",
+    "defaultNotebook": "Brainstorm",
+    "noNotebook": "Aucun carnet",
+    "launching": "Lancement…",
+    "launch": "Brainstormer",
+    "noteCreatedIn": "Note créée dans"
   },
   "byokSettings": {
     "title": "Vos clés API (BYOK)",