[Task] Onramp: Periodic newsletter signup prompt

[samuv]

Summary

Show a newsletter signup prompt periodically — on app launch or after an update — with a cooldown to avoid showing it too frequently.

Details

• Trigger: app restart or update (electron app-update events or startup hook)
• Cooldown: don't show more often than X days (stored in electron-store or localStorage)
• UI: toast or small dialog using existing Sonner toast (renderer/src/routes/__root.tsx) or Dialog primitives (renderer/src/common/components/ui/dialog.tsx)
• Dismissible: track dismiss, impression, and signup events via trackEvent (renderer/src/common/lib/analytics.ts)
• Feature flag support via renderer/src/common/hooks/use-experimental-features.tsx if needed

Acceptance Criteria

• Prompt appears on app launch/update (not more often than cooldown period)
• Cooldown state is persisted across sessions
• Prompt is dismissible
• Impression, signup, and dismiss events are tracked
• Visually consistent with brand