Showcase PreviewMD in the README

[ashtree74]

Summary

• add a full-width product screenshot near the top of the README
• add a dedicated split-view section showing source, math, code, callouts, and diagrams
• make both images clickable at full resolution with descriptive alt text and concise captions

Verification

• copied images are byte-identical to the supplied originals
• both PNGs decode at 2588 × 2296
• README local image references resolve
• git diff --check passes
• visual review found no local paths, user names, account details, or private document content

Docs

README-only presentation change; application behavior and release artifacts are unchanged.

Signed-off-by: Adam Jesionkiewicz adam@jesion.pl

[ashtree74]

Self-review complete. No blocking findings.

Verified:

• both supplied PNGs are byte-for-byte copies of the originals;
• both decode at 2588×2296 and contain no visible private paths, accounts, or user data;
• GitHub loads both assets at full README-column width with preserved aspect ratio;
• links, alt text, captions, and all local README references resolve;
• git diff --check passes.

This is documentation-only and does not change application or release behavior.