chore: README visual assets before OSS release

[rajnavakoti]

Description

The README text has been restructured for impact (centered hero, badges, differentiators near top, cleaner pipeline diagram). But it still looks plain compared to top OSS projects. Visual assets are needed before sharing widely.

Acceptance Criteria

• Logo/banner image added to top of README (centered, dark-friendly)
• Viewer screenshot or GIF showing the UI (at least one page — Digest or Explorer)
• Pipeline diagram as an image (replace ASCII with a clean visual)
• PyPI badge added once package is published to PyPI
• Preview on GitHub confirms everything renders cleanly

Out of Scope

• README text changes (already done in PR chore: redesign README for visual impact #37)
• Viewer functionality changes
• Publishing to PyPI (separate task)