Align extension page with utility-first reference

[yrstm]

What changed

• rebuilt the Mantis Extension page around the supplied Extension Showcase design reference: warm paper/grid background, narrow bordered column, IBM Plex typography, ruled rows, and terminal-like install status
• replaced launch-style artwork and promotional copy with factual usage, permissions, privacy, install status, and limitations
• returned the extension entry on the main Mantis page to the existing light panel treatment
• added regression checks for the canonical URL, honest distribution state, reference design tokens, anchors, and excluded marketing phrases

Why

The first extension page introduced a separate product-launch aesthetic and AI-oriented language that did not fit a lightweight utility. This keeps the product presentation restrained and makes the unavailable store installs explicit.

Impact

This changes GitHub Pages documentation only. Extension capture code, permissions, packages, and Mantis Capture are unchanged. Chrome Web Store and signed Safari/App Store distribution remain deferred.

Validation

• npm test — 72 passing
• npm run types:check
• npm run test:browser — all browser smoke assertions passing
• desktop check at 1440×1000: 880px content column, no horizontal overflow, no missing anchors
• mobile check at 390×844: no horizontal overflow; install, capability, and permission layouts collapse correctly
• Chrome/Safari status switch verified
• no browser or console errors