Blend extension page with Mantis styling

[yrstm]

What changed

• kept the compact layout and information structure from the supplied Extension Showcase reference
• replaced the reference product's warm paper, rust accent, grid, and IBM Plex treatment with the current Mantis white/soft-gray palette, blue accent, system typography, header, and borders
• simplified and shortened the figure frame around the real dark in-page extension panel
• removed external Google Font requests
• updated regression checks for the shared Mantis design tokens

Why

The reference layout was useful, but applying its complete visual identity made the extension page feel separate from the existing Mantis site. This blends the two without returning to the earlier product-launch treatment.

Impact

Documentation styling only. Extension runtime code, permissions, capture behavior, packages, and Mantis Capture are unchanged.

Validation

• npm test — 72 passing
• npm run types:check
• npm run test:browser — all browser smoke assertions passing
• desktop check at 1440×1000: no horizontal overflow, no missing anchors
• mobile check at 390×844: no horizontal overflow; compact label and responsive layouts verified
• Chrome/Safari status switch verified
• no browser or console errors