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/callback/route.ts

@@ -0,0 +1,38 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { getGoogleOAuthCredentials, readIntegrationMeta, writeIntegrationMeta } from '@/lib/integrations/google-integration-tokens'
+
+export async function GET(req: NextRequest) {
+  const url = new URL(req.url)
+  const code = url.searchParams.get('code')
+  const userId = url.searchParams.get('state')
+  const error = url.searchParams.get('error')
+
+  if (error || !code || !userId) {
+    return NextResponse.redirect(`${url.origin}/settings/integrations?error=gmail_auth_failed`)
+  }
+
+  const { clientId, clientSecret } = getGoogleOAuthCredentials()
+  const tokenRes = await fetch('https://oauth2.googleapis.com/token', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+    body: new URLSearchParams({
+      client_id: clientId,
+      client_secret: clientSecret,
+      code,
+      redirect_uri: `${url.origin}/api/integrations/gmail/callback`,
+      grant_type: 'authorization_code',
+    }),
+  })
+
+  const tokenData = await tokenRes.json()
+  if (!tokenData.access_token) {
+    return NextResponse.redirect(`${url.origin}/settings/integrations?error=gmail_token_failed`)
+  }
+
+  const meta = await readIntegrationMeta(userId)
+  meta.gmailAccessToken = tokenData.access_token
+  if (tokenData.refresh_token) meta.gmailRefreshToken = tokenData.refresh_token
+  await writeIntegrationMeta(userId, meta)
+
+  return NextResponse.redirect(`${url.origin}/settings/integrations?connected=gmail`)
+}