feat: implement agent scheduled execution with cron and time picker

- Add scheduledTime, scheduledDay, timezone fields to Agent schema
- Create calculateNextRun() helper with timezone-aware scheduling
- Add POST /api/cron/agents endpoint for external scheduler
- Calculate nextRun on agent create, update, and after execution
- Add time/day picker in agent form (daily/weekly/monthly)
- Show "Next run" countdown in agent card
- Add i18n keys for schedule UI (FR + EN)

External scheduler (N8N, Vercel Cron) should call /api/cron/agents
every 5-15 min. Requires `prisma db push` to apply schema changes.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/app/(main)/agents/agents-page-client.tsx

@@ -120,6 +120,9 @@ export function AgentsPageClient({
       maxSteps: formData.get('maxSteps') ? Number(formData.get('maxSteps')) : undefined,
       notifyEmail: formData.get('notifyEmail') === 'true',
       includeImages: formData.get('includeImages') === 'true',
+      scheduledTime: (formData.get('scheduledTime') as string) || undefined,
+      scheduledDay: formData.get('scheduledDay') ? Number(formData.get('scheduledDay')) : undefined,
+      timezone: (formData.get('timezone') as string) || undefined,
     }
 
     if (editingAgent) {