feat(meta): add the Open Graph preview image

[kNoAPP]

Description

This finishes the in-repo part of #451. #456 added the Open Graph and Twitter metadata but had no image to point at. #458 then added the social card artwork. This PR connects the two, so a kn0.app link unfurls with the large card.

app/opengraph-image.png: the #458 social card (1200×630, 86 KB), moved here from public/brand/social-card.png using Next's static-file convention. It was moved rather than copied so there's only one copy of the exported PNG.

app/opengraph-image.alt.txt: the alt text for the card. The file has no trailing newline on purpose: Next reads it verbatim, so a newline would end up inside the content attribute.

public/brand/README.md: the social-card.svg row now says where its export lives, the separate social-card.png row is gone, and the export command writes the card to app/opengraph-image.png. I re-ran the command: both PNGs came out byte for byte identical to the committed files.

No twitter-image.png. Next copies the Open Graph image into the Twitter tags on its own, so X gets twitter:image without a second file.

Verification

npm run build, then in out/index.html:

<meta property="og:image" content="https://kn0.app/opengraph-image.png?opengraph-image.3zoje1vxv8dc2.png"/>
<meta property="og:image:type" content="image/png"/>
<meta property="og:image:width" content="1200"/>
<meta property="og:image:height" content="630"/>
<meta property="og:image:alt" content="MeshCore Desktop: message, map and manage your MeshCore radio from the browser."/>
<meta name="twitter:card" content="summary_large_image"/>
<meta name="twitter:image" content="https://kn0.app/opengraph-image.png?opengraph-image.3zoje1vxv8dc2.png"/>
<meta name="twitter:image:alt" content="MeshCore Desktop: message, map and manage your MeshCore radio from the browser."/>
<meta name="twitter:image:type" content="image/png"/>
<meta name="twitter:image:width" content="1200"/>
<meta name="twitter:image:height" content="630"/>

og:title, og:description, og:url, twitter:card and theme-color are still there from #456. out/opengraph-image.png is exported, and the query string on its URL is a content hash that changes whenever the image does, so re-exporting the card also busts crawler caches.

Spell-check, lint, type-check and build all pass. format:check passes once line endings are ignored; locally it only fails on the Windows CRLF checkout. There's no visible UI change, so there are no screenshots, and no radio behavior is involved.

Theme color: #446 hasn't added the per-scheme theme-color entries yet, so the page still has only the plain #4f8ef7 entry from #456, and that's the one Discord reads. Once #446 lands, the comment on viewport.themeColor in app/layout.tsx covers keeping that entry first.

After merge (can't be done from the repo)

• After deploy, paste https://kn0.app/?v=2 into Discord and check the title, description, large card and blue side bar.
• Check https://www.opengraph.xyz and LinkedIn Post Inspector.
• Maintainer: upload public/brand/repo-social.png under Settings → General → Social preview (there's no API for it), then check that a repo link pasted into Discord shows the card.

Issues

• Closes feat(meta): rich link previews for kn0.app in Discord and other apps #451

Type of Change

• ❌ ! -- Breaking change (fix or feature that would cause existing
  functionality to change)
• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• 📝 docs -- Additional or updated documentation
• 🎨 style -- Code style changes (formatting, missing semi colons, etc)
• 🧹 refactor -- Code refactor
• ⚡ perf -- Performance improvement
• 🧪 test -- Tests (adding or updating tests)
• 🏗️ build -- Build system or external dependencies changes
• ✅ ci -- Build configuration change
• 🗑️ chore -- Grunt tasks
• ⏪ revert -- Revert previous changes