feat(site): redesign homepage and extensions experience

[jouwdan]

The homepage now presents Meith as open-source, self-hosted forum software through a restrained editorial layout, clear ownership and deployment copy, and a compact live theme preview.

• Keep the hero focused on the headline, introduction and setup/demo actions; retain detailed technology descriptions with more visible repository-derived versions.
• Render actual forum components for Default, Clubhouse, Phasebook, Raidframe and Midnight. The preview starts from the site colour preference, then switches independently; native theme controls also work without JavaScript.
• Place the half-width preview beside theme, plugin and API guides, and keep Clubhouse counts inline at narrow widths.
• Move catalogue pages to /extensions, add them to both navigation menus, and move appearance controls to the footer. Redirect old catalogue links while preserving /marketplace/v1.json and screenshot URLs.
• Keep the catalogue’s closing actions secondary and document the updated layout and behaviour.

Validation:

• pnpm verify
• pnpm site:build
• Marketing Playwright suite: 9 tests passed, covering desktop/mobile layout, navigation, redirects, all five themes in both palettes, JavaScript-disabled controls and colour-preference independence.
• pnpm comments:check and git diff --check
• Visual review on desktop and phone in both colour schemes.