seo: add social preview image and fix Open Graph / Twitter meta

Link previews (LinkedIn, X, etc.) were broken: og:image pointed at a
GitHub-hosted favicon (.ico, too small, likely 404) and og:url pointed at
the GitHub repo instead of the live site.

- Add a branded 1200x630 social card at assets/og-image.png.
- Point og:url/canonical at https://securebit.chat/ and og:image at the new
  PNG via absolute URLs; add og:image:width/height/type, og:site_name,
  og:image:alt and twitter:image.
- Refresh the meta description to match the product (no longer version-stamped).

meta.json

@@ -1,10 +1,10 @@
 {
-  "version": "1782329449554",
-  "buildVersion": "1782329449554",
+  "version": "1782331061287",
+  "buildVersion": "1782331061287",
   "appVersion": "4.9.1",
-  "buildTime": "2026-06-24T19:30:49.591Z",
-  "buildId": "1782329449554-cbf5c5f",
-  "gitHash": "cbf5c5f",
+  "buildTime": "2026-06-24T19:57:41.339Z",
+  "buildId": "1782331061287-96fffab",
+  "gitHash": "96fffab",
   "generated": true,
-  "generatedAt": "2026-06-24T19:30:49.593Z"
+  "generatedAt": "2026-06-24T19:57:41.349Z"
 }