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/components/dashboard-widget-help.tsx

@@ -0,0 +1,57 @@
+'use client'
+
+import { useState } from 'react'
+import { HelpCircle, X } from 'lucide-react'
+import { useLanguage } from '@/lib/i18n'
+import type { DashboardWidgetId } from '@/lib/dashboard/layout'
+
+interface DashboardWidgetHelpProps {
+  widgetId: DashboardWidgetId
+  className?: string
+}
+
+export function DashboardWidgetHelp({ widgetId, className = '' }: DashboardWidgetHelpProps) {
+  const { t } = useLanguage()
+  const [open, setOpen] = useState(false)
+  const helpKey = `homeDashboard.widgetHelp.${widgetId}`
+  const text = t(helpKey)
+
+  if (text === helpKey) return null
+
+  return (
+    <div className={`relative ${className}`}>
+      <button
+        type="button"
+        onClick={() => setOpen(v => !v)}
+        className={`p-1 rounded-full border transition-colors ${
+          open
+            ? 'border-brand-accent/40 bg-brand-accent/10 text-brand-accent'
+            : 'border-border/30 bg-white/90 dark:bg-zinc-900/90 text-concrete hover:text-brand-accent hover:border-brand-accent/30'
+        }`}
+        aria-label={t('homeDashboard.widgetHelpLabel')}
+        aria-expanded={open}
+      >
+        <HelpCircle size={12} strokeWidth={2} />
+      </button>
+
+      {open && (
+        <div className="absolute top-full end-0 mt-1.5 z-30 w-[min(280px,calc(100vw-3rem))] p-3 rounded-xl border border-brand-accent/20 bg-white dark:bg-zinc-900 shadow-lg">
+          <div className="flex items-start justify-between gap-2 mb-1">
+            <p className="text-[8px] font-mono font-bold uppercase tracking-wider text-brand-accent">
+              {t(`homeDashboard.widgets.${widgetId}`)}
+            </p>
+            <button
+              type="button"
+              onClick={() => setOpen(false)}
+              className="text-concrete hover:text-ink shrink-0"
+              aria-label={t('homeDashboard.widgetHelpClose')}
+            >
+              <X size={10} />
+            </button>
+          </div>
+          <p className="text-[10px] text-concrete leading-relaxed">{text}</p>
+        </div>
+      )}
+    </div>
+  )
+}