fix(dashboard): audit Second Brain — pistes, erreur briefing, dark, crédits

Corrige loadPaths sans argument après Echo/IA, bannière si briefing en échec,
checklist connexion inversée, skeletons dark, libellé usage = solde unique.

memento-note/locales/en.json

@@ -4218,7 +4218,9 @@
     "widgetAgentActivityEmpty": "No agent activity in the last 48 hours.",
     "widgetPinnedEmpty": "No pinned notes yet.",
     "widgetActivityHint": "Notes edited over the last 90 days.",
-    "widgetUsageHint": "Monthly AI usage by feature.",
+    "widgetUsageHint": "Single AI credit balance (all features).",
+    "briefingLoadError": "Could not load the dashboard.",
+    "briefingRetry": "Retry",
     "widgetCategories": {
       "essential": "Essentials",
       "paths": "Paths & review",