docs: add README hero screenshot previews

[neezzzy]

Summary

• Adds a hero screenshot preview section to the README for a more portfolio-ready first impression.

Changes

• Added two README preview placeholder assets for dashboard and workflow views.
• Updated the README to show the hero previews near the top of the project page.
• Kept the screenshots clearly labeled as placeholders until finalized app captures are available.

Testing

• npm run typecheck
• npm test

Notes

• No application code changed.
• git diff --check passed locally.
• The preview assets are intentionally placeholders and can be replaced later with real UI screenshots.