seo: rework title/description, add structured data, refine social card

- New keyword-focused <title> and meta description.
- Add robots (index, follow, max-image-preview:large) and og:locale.
- Add schema.org JSON-LD (WebSite + WebApplication) with feature list,
  free/MIT offer and GitHub sameAs — non-executable data block, so it
  passes the strict script-src CSP.
- Regenerate the 1200x630 social card without the redundant URL line.

meta.json

@@ -1,10 +1,10 @@
 {
-  "version": "1782331061287",
-  "buildVersion": "1782331061287",
+  "version": "1782331597042",
+  "buildVersion": "1782331597042",
   "appVersion": "4.9.1",
-  "buildTime": "2026-06-24T19:57:41.339Z",
-  "buildId": "1782331061287-96fffab",
-  "gitHash": "96fffab",
+  "buildTime": "2026-06-24T20:06:37.081Z",
+  "buildId": "1782331597042-ae6900d",
+  "gitHash": "ae6900d",
   "generated": true,
-  "generatedAt": "2026-06-24T19:57:41.349Z"
+  "generatedAt": "2026-06-24T20:06:37.090Z"
 }