feat(seo): add Google Preferred Sources button + footer link

New self-contained preferred-source.js (referenced site-wide via
seo_inject GA_LINES):
- small "Add us on Google" text link in the footer (.legal-links) on
  every page, trilingual, using the domain deeplink;
- Google's official interactive pill (publisher.js) on the homepage,
  News hub and Blog hub only, localized (en/mk/sl).
Lets readers mark MSOS as a preferred source (Top Stories / Discover /
News / AI surfaces). Cache-bust bump included.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tools/seo_inject.py

@@ -196,6 +196,7 @@ GA_LINES = [
     '    <script src="/consent.js" defer></script>',
     '    <!-- Cloudflare Web Analytics (cookieless, no consent needed) -->',
     "    <script type=\"module\" src=\"https://static.cloudflareinsights.com/beacon.min.js\" data-cf-beacon='{\"token\": \"eff2ed428ab3495eb9d4605234f226e8\"}'></script>",
+    '    <script src="/preferred-source.js" defer></script>',
 ]
 
 # ---- page trees (for hreflang parity) ----