docs(site): add screenshots with alt text and image metadata

[crypto-frog]

Summary

Adds three screenshots of Darkkle running against a blank spreadsheet, with alt text, explicit dimensions, and image structured data.

Image	Shows
01-dark-grid.png	Themed grid, toolbar, menu bar, sheet tabs; readable column letters and row numbers
02-settings-panel.png	Pinned settings panel, on/off switch, live preview, quick styles
03-tinted-selection.png	Range D7:E11 with coordinated green tint on row and column header highlights

Privacy handling

The original captures contained material that must not enter a public repository:

• the spreadsheet document ID in the address bar;
• the browser bookmarks bar;
• the signed-in account avatar;
• the desktop taskbar.

SECURITY.md and AGENTS.md both prohibit committing document IDs and unredacted screenshots, and removing them after the fact would require history rewriting, which is also prohibited.

Each capture was cropped to the application area and the avatar corner was masked. The remaining content is a blank Untitled spreadsheet. The Rent / Power / Total table visible in the second image is the extension's own built-in preview widget, not spreadsheet content.

Image and search metadata

• Descriptive alt text on every image, written for screen readers first and usable as image-search text.
• Explicit width and height so no layout shift occurs while loading, plus loading="lazy" and decoding="async" below the fold.
• figcaption explaining what each image demonstrates.
• screenshot array of ImageObject added to the existing SoftwareApplication JSON-LD.
• The overview page now declares a screenshot as its og:image. The previous default was the 128px icon, which was too small for the summary_large_image Twitter card the site already emits.

Images are 1500px wide, 90–199 KB each.

Testing

• npm run check passes. docs/ is not in scripts/release-files.mjs, so the extension archive and checksum are unchanged.
• Documentation-only change.
• Rendered output needs checking after merge, since the Jekyll build only runs against main.