website: restore the index grid background

Git archaeology: the grid was never a deliberate removal. It lived as a fixed
`body::before` in index.html's inline <style> block and was dropped when bed8675
("D3 Part 2: index + kapcsolat on design system v2") migrated the page onto
site.css. dd54e4c, which created site.css, has no body::before — it was a
porting omission, and nothing took its place. No asset was lost; it was pure CSS.

Restored at its original geometry and subtlety (50px cells, 1px lines, 3%), not
redesigned. Only change: the accent is the v2 --blue #0083D8 instead of the
retired legacy #0088cc, which site_gates.py bans. Scoped to body.page-index
because index is the only page that ever had it.

site.css cache-bust bumped to v=2 across all seven pages (nginx caches 7d).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nn3VgQk9iwEGgyx6QJ2NvE

website/technologiak.html

@@ -35,7 +35,7 @@
     }
     </script>
     <!-- shared assets: bump ?v= together on any site.css/icons.svg change -->
-    <link rel="stylesheet" href="/assets/site.css?v=1">
+    <link rel="stylesheet" href="/assets/site.css?v=2">
     <link rel="icon" type="image/png" href="/assets/logo_favicon_2.svg">
     <script defer src="https://stats.felhom.eu/script.js" data-website-id="d419db57-5e1c-48df-bdb3-c29f25fb1661"></script>
 </head>