feat(meta): add Open Graph and Twitter link-preview metadata

[kNoAPP]

Description

A kn0.app link pasted into Discord, Slack, X or LinkedIn gets a bare link today. The page metadata had no Open Graph or Twitter tags and no metadataBase. This PR adds the metadata half of #451. The preview image comes in a follow-up, because it's exported from #450's social card and that artwork doesn't exist yet.

app/layout.tsx

• metadataBase: new URL('https://kn0.app') (matches public/CNAME), so the image URL added later comes out absolute.
• openGraph: type: 'website', siteName, title, description, url: '/', locale: 'en_US'.
• twitter: { card: 'summary_large_image' }. Next fills twitter:title and twitter:description from the Open Graph fields, so they aren't repeated.
• viewport.themeColor: '#4f8ef7' (dark-theme --accent). This is a plain theme-color with no media attribute, which Discord uses to color the embed's side bar. A comment asks feat(pwa): make the app installable as a desktop app #446 to keep it as the first entry when it adds the per-scheme light and dark entries.
• New description, 117 characters: "Message, map and manage your MeshCore LoRa radio from the browser over USB, Bluetooth or WiFi. No account, no server."
• The app name and description live in one constant each (APP_NAME and APP_DESCRIPTION), so a rename touches one line.

public/manifest.webmanifest: now has the same description, so the install prompt and the link preview say the same thing.

Verification

npm run build → out/index.html contains:

<meta name="theme-color" content="#4f8ef7"/>
<meta name="description" content="Message, map and manage your MeshCore LoRa radio …"/>
<meta property="og:title" content="MeshCore Desktop"/>
<meta property="og:description" content="Message, map and manage …"/>
<meta property="og:url" content="https://kn0.app/"/>
<meta property="og:site_name" content="MeshCore Desktop"/>
<meta property="og:locale" content="en_US"/>
<meta property="og:type" content="website"/>
<meta name="twitter:card" content="summary_large_image"/>
<meta name="twitter:title" content="MeshCore Desktop"/>
<meta name="twitter:description" content="Message, map and manage …"/>

Lint, type-check, spell-check and build all pass. There's no visible UI change, so there are no screenshots, and no radio behavior is involved.

Still to do for #451 (after #450 lands)

• Add app/opengraph-image.png (1200×630, well under 1 MB) with opengraph-image.alt.txt. og:image, its width, height and alt, and the X image will then appear automatically.
• After deploy, paste the link into Discord and check it with opengraph.xyz and LinkedIn Post Inspector.
• The maintainer uploads the repo-social PNG under the repository's Settings → Social preview.

Heads-up: a plain accent theme-color also tints browser chrome in browsers that honor it, like Safari's tab bar and mobile Chrome. That's the trade-off the ticket chose to get the Discord accent.

Issues

• Part of feat(meta): rich link previews for kn0.app in Discord and other apps #451. The image half is blocked on design: create original brand artwork (logo mark, app icon, social cards) #450, so this PR leaves the issue open.

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