…sh, canonical, alt text

Closes #2143 — Dynamic OG images for homepage, explore, and collections
- apps/web/app/opengraph-image.tsx: 1200x630 dark-theme OG for homepage
- apps/web/app/explore/opengraph-image.tsx: 1200x630 OG for Data Explorer
- apps/web/app/collections/[slug]/opengraph-image.tsx: updated to 1200x630
  dark theme (#1a1a1b bg, #f7df83 accent); fetches top repos dynamically
  and switched runtime to nodejs to support @napi-rs/canvas DB calls
- Copied Poppins fonts to app/ and explore/ route segments

Closes #2140 — SoftwareApplication JSON-LD schema on homepage
- components/json-ld.tsx: added SiteApplicationJsonLd component
  (type: SoftwareApplication, applicationCategory: DeveloperApplication,
  operatingSystem: Web, offers: free / $0)
- app/page.tsx: renders <SiteApplicationJsonLd /> above FAQPage schema

Closes #2137 — Canonical URLs to prevent duplicate content
- app/collections/page.tsx: added alternates.canonical: '/collections'
  so paginated/filtered variants (?page=, ?sort=, ?q=) do not generate
  duplicate indexed content
- Explore page already had canonical; trending page already had canonical

Closes #2121 — Custom 404 page
- app/not-found.tsx: dark theme 404 with helpful links to Home,
  Data Explorer, Trending, and Collections; robots noindex

Closes #2118 — Trailing slash consistency
- next.config.mjs: trailingSlash: false (explicit, matches Vercel default)
  ensures /path/ redirects to /path consistently

Closes #2135 — Image SEO: alt text + image sitemap
- app/home-content.tsx: fixed empty alt on repo owner avatar
- app/sitemap.ts: added image entries for homepage (seo-widgets-homepage.jpeg)
  and per-collection opengraph-image routes; static pages now use canonical
  paths without trailing slash