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/migrations/20260714170000_add_gmail_scan_history/migration.sql

@@ -0,0 +1,21 @@
+-- CreateTable
+CREATE TABLE "GmailScanHistory" (
+    "id" TEXT NOT NULL,
+    "userId" TEXT NOT NULL,
+    "messageId" TEXT NOT NULL,
+    "category" TEXT NOT NULL,
+    "data" JSONB,
+    "noteId" TEXT,
+    "scannedAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "GmailScanHistory_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "GmailScanHistory_userId_scannedAt_idx" ON "GmailScanHistory"("userId", "scannedAt");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "GmailScanHistory_userId_messageId_key" ON "GmailScanHistory"("userId", "messageId");
+
+-- AddForeignKey
+ALTER TABLE "GmailScanHistory" ADD CONSTRAINT "GmailScanHistory_userId_fkey" FOREIGN KEY ("userId") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE;