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/app/api/integrations/gmail/route.ts

@@ -0,0 +1,67 @@
+/**
+ * Google Gmail Integration (OAuth + scan trigger)
+ */
+
+import { NextRequest, NextResponse } from 'next/server'
+import { auth } from '@/auth'
+import {
+  getGoogleOAuthCredentials,
+  getGoogleTokens,
+  readIntegrationMeta,
+  writeIntegrationMeta,
+} from '@/lib/integrations/google-integration-tokens'
+import { gmailScannerService } from '@/lib/integrations/gmail-scanner.service'
+
+const GMAIL_SCOPE = 'https://www.googleapis.com/auth/gmail.readonly'
+const GOOGLE_AUTH_URL = 'https://accounts.google.com/o/oauth2/v2/auth'
+
+function redirectUri(origin: string) {
+  return `${origin}/api/integrations/gmail/callback`
+}
+
+export async function GET(req: NextRequest) {
+  const session = await auth()
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+  }
+
+  const url = new URL(req.url)
+  const userId = session.user.id
+
+  if (url.searchParams.get('connect') === '1') {
+    const { clientId } = getGoogleOAuthCredentials()
+    if (!clientId) {
+      return NextResponse.json({ error: 'Google OAuth not configured' }, { status: 503 })
+    }
+    const authUrl = new URL(GOOGLE_AUTH_URL)
+    authUrl.searchParams.set('client_id', clientId)
+    authUrl.searchParams.set('redirect_uri', redirectUri(url.origin))
+    authUrl.searchParams.set('response_type', 'code')
+    authUrl.searchParams.set('scope', GMAIL_SCOPE)
+    authUrl.searchParams.set('access_type', 'offline')
+    authUrl.searchParams.set('prompt', 'consent')
+    authUrl.searchParams.set('state', userId)
+    return NextResponse.redirect(authUrl.toString())
+  }
+
+  if (url.searchParams.get('scan') === '1') {
+    const result = await gmailScannerService.scanUser(userId)
+    return NextResponse.json(result)
+  }
+
+  const status = await gmailScannerService.getStatus(userId)
+  return NextResponse.json(status)
+}
+
+export async function DELETE() {
+  const session = await auth()
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 })
+  }
+
+  const meta = await readIntegrationMeta(session.user.id)
+  delete meta.gmailAccessToken
+  delete meta.gmailRefreshToken
+  await writeIntegrationMeta(session.user.id, meta)
+  return NextResponse.json({ success: true })
+}