site: restore viewer, collaboration, and comparison pages

[matejcsok-pcb]

The landing-page redesign originally included three complete intent pages, but their source was parked outside Git before the landing page shipped. This restores those pages on top of the current staging branch.

What changed

• Add the /viewer page for opening and sharing KiCad projects in a browser.
• Add the /collaboration page covering live presence, follow mode, comments, invite links, and roles.
• Add /compare plus dedicated KiCad, EasyEDA, and Flux comparison routes.
• Restore the comparison data, source citations, switcher, table, and embedded Gerber demo components.
• Link the pages from the landing page and footer.
• Apply scoped COOP/COEP headers to comparison routes for the embedded Gerber viewer.
• Include every new route in the generated sitemap.

Validation

• npm run check — 0 errors
• npm test — 7 tests passed
• npm run build — 23 pages generated, including all restored routes