feat(dashboard): tableau de bord Second Brain configurable avec chargement progressif

Briefing granulaire, pistes rapides puis enrichissement async, layout persisté v5,
suggestions agents, intégration Gmail et navigation sidebar alignée sur /home.

Co-authored-by: Cursor <cursoragent@cursor.com>

memento-note/prisma/schema.prisma

@@ -30,6 +30,7 @@ model User {
   onboardingStep        Int                     @default(0)
   accounts              Account[]
   agents                Agent[]
+  agentSuggestions      AgentSuggestion[]
   aiFeedback            AiFeedback[]
   brainstormActivities  BrainstormActivity[]
   brainstormParticipant BrainstormParticipant[]
@@ -58,6 +59,7 @@ model User {
   bridgeSuggestions     BridgeSuggestion[]
   flashcardDecks        FlashcardDeck[]
   errorLogs             ErrorLog[]
+  gmailScanHistory      GmailScanHistory[]
 }
 
 model Account {
@@ -370,6 +372,7 @@ model UserAISettings {
   autoSave             Boolean @default(true)
   aiProcessingConsent  Boolean @default(false)
   svgComplexity        String  @default("simple")
+  dashboardLayout      Json?
   integrationTokens    Json?   // Stores third-party integration tokens (Readwise, Calendar, etc.)
   user                 User    @relation(fields: [userId], references: [id], onDelete: Cascade)
 
@@ -480,6 +483,39 @@ model AgentAction {
   @@index([agentId])
 }
 
+model AgentSuggestion {
+  id                 String   @id @default(cuid())
+  userId             String
+  topic              String
+  reason             String
+  suggestedRole      String   @db.Text
+  suggestedType      String   @default("researcher")
+  suggestedFrequency String   @default("weekly")
+  relatedNoteIds     String   // JSON string[]
+  clusterId          Int?
+  status             String   @default("pending") // pending | accepted | dismissed
+  createdAt          DateTime @default(now())
+  updatedAt          DateTime @updatedAt
+  user               User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId, status])
+  @@unique([userId, clusterId])
+}
+
+model GmailScanHistory {
+  id        String   @id @default(cuid())
+  userId    String
+  messageId String
+  category  String   // flight | package | subscription
+  data      Json?
+  noteId    String?
+  scannedAt DateTime @default(now())
+  user      User     @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@unique([userId, messageId])
+  @@index([userId, scannedAt])
+}
+
 model Conversation {
   id              String        @id @default(cuid())
   title           String?