[Demo] Add HUD, Obsidian daily note, and privacy architecture screenshots

[Longfellow1]

Background

KeyPulse needs visual proof. Screenshots help users quickly understand what the menu bar HUD looks like, what the Obsidian output looks like, and why the privacy model is architecture-level rather than policy-only.

Task

Add clean, privacy-safe visual assets to the README.

Acceptance criteria

• HUD screenshot uses a clean demo state
• Obsidian daily note screenshot uses mock data only
• Obsidian weekly note screenshot uses mock data only
• English daily/weekly screenshots are available for the English README or external launch posts
• Privacy architecture image shows filtering before persistence
• README renders all images correctly on GitHub